Intelligence of troops infected with hookworm vs. those not infected

Ill

Intelligence of Troops Infected with Hookworm
vs. Those not Infected

BY
GARRY C. MYERS

Head of Department of Psychology, School of Education, Cleveland, Ohio

Reprinted from THE PEDAGOGICAL SEMINARY
October, 1920, Vol. XXVII, pp. 211-242

B10UC
UBR"

INTELLIGENCE OF TROOPS INFECTED WITH
HOOKWORM VS. THOSE NOT INFECTED

By GARRY C. MYERS
Head of Dept. of Psychology, School of Education, Cleveland, O.

HISTORY AND PROCEDURE

During the War, Major B. F. Pittenger, Sanitary Corps,
chief psychological examiner at Camp Sevier, in collaboration
with Major Charles A. Kofoid, Sanitary Corps, in charge of
the Laboratory Car Metchnikoff, made a study of the com-
parative intelligence ratings of the recruits found to have
hookworm infection and those without infection, of the 9,254
men of the April, 1918, draft increment. A report thereon
to the Office of the Surgeon General, Division of Psychology,
was made by Major Pittenger in comparative tables for hook-
worm and non-hookworm groups, respectively, in terms of
Alpha and Beta scores, for white and for colored troops. Of
these data Major Charles A. Kofoid and Lieutenant J. P.
Tucker, Medical Corps, offer some interpretation in their re-
port to the Laboratory Division of the Surgeon General's
Office " on the relationship of infection by hookworm to the
incidence of morbidity in 22,842 men in the United States
Army at Camp Bowie, Texas, from October, 1917, to April,
1918."

The manifest inferiority of the median intelligence scores
of the 762 hookworm cases to the 8,492 non-hookworm cases
suggested the desirability of a more comprehensive study
wherein each hookworm case could be paired with a non-
hookworm case from the same local community. Accordingly
Major Kofoid recommended to the Surgeon General a co-

43C2S2.

212 fyt^ftljCfeNt'E'OF JTRpQPS INFECTED WITH HOOKWORM

operative study of the problem by the Laboratory Division
and the Section of Psychology. A psychological officer was
not immediately available, but Major John L. Riley, Sanitary
Corps, was released by the Section of Physical Reconstruction
for temporary duty with Major Kofoid in organizing and
promoting the early work of this investigation. Under their
direction a group of officers, laboratory technicians and en-
listed men proceeded to assemble and to code for Hollerith
treatment the intelligence ratings of recruits infected with
hookworm and of their non-infected pairs.

During the second week in July, 1919, Captain Garry C.
Myers of the psychological personnel became available for
duty on this investigation. He reported to the Surgeon Gen-
eral to select and ship records from the files of the Section of
Psychology to the Port of Embarkation, where initial work
was already in progress. A week later he was ordered to
U. S. A. Laboratory, Port of Embarkation, New York City,
for duty in connection with the statistical treatment of the
records.

The personnel at the laboratory, in charge of Lieut. Col.
Edwin H. Schorer, varied from seventeen officers, enlisted
men and women during July and early August to five officers
and assistants in early September. Different officers from
time to time awaiting their discharge were assigned for but a
few days, whereas Second Lieutenant Conrad Erwin Ronnen-
berg, Sanitary Corps, and Lieutenant Henry S. Weigle, Medi-
cal Corps, were indispensable collaborators throughout the
whole period of the work at the port of embarkation.

Major Kofoid left the service on August 8th and Major
Riley left the service on September 6th, leaving the work in
charge of Captain Myers, who again reported at the office of
the Surgeon General on September 10th to complete the study.
Upon his arrival, it was found that inordinate demands upon
the statistical department eliminated all hope of final Hollerith
tabulation of the data and it appeared that completion of the
study would have to be postponed indefinitely. However,
through the interest and initiative of Colonel Joseph F. Siler,
Medical Corps, of the Laboratory Division and Colonel F. F.
Russell, Medical Corps, of the Army Medical School, two
enlisted men were assigned to assist Captain Myers in assem-
bling the data by the traditional method of hand tallying.
Frequency tables were thus completed in about three weeks.
Sergeant Benjamin M. Oppenheim was then loaned by the
Division of Coordination, Organization and Equipment to plot
the graphs.

Thus it will be perceived that Major Kofoid initiated the

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 213

study and the larger machinery for its execution; Major Riley
supervised the greater portion of the work assembling the
data for this study at the port embarkation; and Captain
Myers is responsible for the final assembling and treatment
of data with their interpretations.

The methods of pulling intelligence rating cards was as fol-
lows : Each time a card of a hookworm soldier was found,
another card was paired with it, of a soldier, reported on
hookworm rosters as not having hookworm, and whose home
address was of the same county as that of the hookworm case
with whose card it was paired.

The study was started with the hope to compare 10,000
hookworm cases paired with 10,000 non-hookworm cases from
the same local areas in terms of home addresses. However,
owing to the frequent disparity between the groups and organ-
izations surveyed for hookworm and the groups and organiza-
tions surveyed by the intelligence rating boards (since the
latter boards were organized in some camps subsequent to the
earlier hookworm surveys) and to the absence of necessary
data either from the hookworm rosters or from the intelli-
gence rating cards, only 6,639 hookworm and 6,639 non-hook-
worm cases were studied, of which number of each there were
612 cases of colored troops.

In every instance where two or more examinations had been
given, that score was counted which on the basis of the equiva-
lent mental age scale as per the Psychological Examiner's
Guide, page 91, gave the highest record. All Alpha and all
Beta records were assembled in separate tables and all indi-
vidual-tests records were merged with Alpha and Beta scores
into a common scale on the basis of equivalent mental ages,
which in turn were converted into equivalent letter ratings as
per Examiner's Guide, page 91.

The statisticians1 in the Section of Psychology of the Sur-
geon General's Office who had developed these mental age
and letter rating equivalents to meet an urgent need during
the war, had found in a later study that these equivalents were
but roughly accurate. Consequently they, with the help of
Karl Pearson, developed by an elaborate and exhaustive study
the groundwork of very refined equivalents. However, these
statisticians left the service before a table was developed
applicable for this study. Certainly then the ratings of this
study, which are indicated by the " Common Scale," are not
so exact as they ought to be ; yet they are the best which the
available standards offered as expedient.

1 Mr. Carl R. Brown and Lt. Mark A. May.

m0s?& ) ^Yrl'

•'

214 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

In addition to the intelligence rating the chronological age
and years of schooling were taken into account. Rank also
was noted early in the study but because the intelligence record
cards — as would be expected since they were taken soon after
the recruits entered camp — listed practically all soldiers as
privates, rank difference proved sterile for this study.

Furthermore, the record cards were pulled on the basis of
" heavy territory " and " light territory," the former meaning
a county in which the army hookworm survey indicated more
than 10% of recruits infected with hookworm; the latter, less
than 10% of recruits infected. In case, then, an intelligence
record card was found of a soldier infected with hookworm,
whose home address was in a light county, the intelligence
record card of a non-hookworm soldier from the same county
was pulled.

With the negroes, whose intelligence rating cards as well
as hookworm survey rosters were sometimes designated as
merely from Texas, say, or Tennessee, it was almost impos-
sible to compare light territory and heavy territory groups.
This fact, in addition to the relatively small number of cases,
explains why the colored troops are limited to one general
table of comparison of hookworm cases with non-hookworm
cases.

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 215

RESULTS
TABLE I

PER CENT DISTRIBUTION OF RATINGS
All White Troops

Alpha

Beta

Com. Scale

Hook-

Non-

Hook-

Non-

Hook-

Non-

worm

hook-

worm

hook-

worm

hook-

worm

worm

worm

A

1.2

3.0

0.7

1.1

1.5

3.5

B

4.4

7.9

1.1

2.2

4.6

8.2

c+

11.5

16.6

2.2

4.2

7.3

10.9

c

26.9

29.6

10.6

14.1

23.7

27.4

c—

30.3

25.2

27.6

25.6

34.8

29.6

D

17.4

11.7

40.1

36.9

19.3

14.5

D—

8.1

5.8

17.4

15.7

8.5

5.9

White Troops (Heavy Territory)

A

1.1

2.6

0.6

0.9

1.3

2 »

B

4.8

7.4

0.9

2.1

4.6

7.3

c+

12.3

17.1

2.2

3.8

7.0

10.3

c

27.0

30.7

10.0

14.0

22.5

25.7

c-

30.7

24.8

27.9

26.0

34.8

29.7

D

16.8

11.3

41.9

38.2

21.0

15. i

D—

7.3

6.1

16.5

15.0

8.8

6.6

White Troops (Light Territory)

A

1.4

3.6

1.2

2.3

1.8

4.7

B

4.0

8.7

1.9

2.5

4.6

9.8

c+

10.4

16.0

2.3

6.0

8.1

12.1

c

26.8

28.1

12.9

15.0

26.2

28.7

c—

29.8

25.8

26.7

24.0

35.0

29.5

D

18.2

12.3

34.2

31.0

16.2

10.8

D—

9.4

5.5

20.8

19.2

8.0

4.4

Colored Troops

A

0.0

0.0

0.7

1.3

0.7

1.6

B

1.8

1.5

0.7

2.9

1.1

2.0

c+

1.8

3.5

1.4

3.6

1.0

2.8

c

6.2

10.4

5.2

5.2

5.0

7.2

c—

17.3

14.8

11.0

11.4

15.2

18.8

D

16.7

22.3

37.2

33.3

38.4

35.9

D—

56.2

47.5

43.8

42.2

38.6

31.7

216 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

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218 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

Table I shows the comparative distribution by letter ratings
of hookworm and non-hookworm groups for Alpha, Beta and
" Common Scale," for all white troops, for white troops of
light territory and of those of heavy territory, and for all
colored troops. Median numerical scores for Alpha and for
Beta, median mental age (Common Scale), median chrono-
logical age and median number of years attending school are
given under corresponding legends in Table II. These dis-
tributions of ratings of ages and years of schooling with their
respective medians are all presented graphically in plates 1-22.

Furthermore, all the hookworm troops are compared with
all the non-hookworm troops in respect to successive 10%
increments in Alpha, Beta and " Common Scale," as shown
in plates 17-22.

Data from which these graphs are constructed are given in
Table III.

ANALYSIS AND INTERPRETATION

By reference to Tables I and II it is found that the intelli-
gence of hookworm troops is lower than for the non-hook-
worm troops; that they have fewer years of schooling; and
that they are younger chronologically. Mere inspection of
the tables show these differences to be unmistakable and to be
based on such a large number of cases as not to warrant com-
putation of a mathematical probable error of these differences.2
Furthermore, all these differences are greater for white troops
from " light " territory than for those from " heavy " terri-
tory and, in most cases, greater for the white troops than for
the colored troops.

Moreover, when, according to Table III and graphs, plates
17-22, hookworm and non-hookworm troops are compared by
average score of successive 10% steps from the highest to the
lowest, it is clear that the difference in intelligence rating
between hookworm and non-hookworm troops of the lowest
level of intelligence is comparatively slight. From this it is
safe to infer that the lower the intellectual scale, the less the
relative inferiority of the hookworm troops.

So much for the data. What do they mean ? Before offer-
ing an interpretation, let there be a critical survey of earlier

2 It should be noted that the sum of the Alpha cases and of the
Beta cases of hookworm and non-hookworm groups, respectively, do
not quite equal the total number for each group in the " Common
Scale" column, since the difference was made up by the individually
examined cases. Likewise the total numbers for the age and schooling
columns do not, as they should, equal the totals of the Common Scale
column, since some troops failed to note their age, some their schooling,
and some, both.

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 219

studies of similar problems. The first study designed to meas-
ure the influence of hookworm disease on intelligence was
made on school children by E. K. Strong under the auspices
of the Rocke!iHler Foundation, in 1916.3
From his study, Strong concludes:

1. " Hookworm disease interferes very radically with men-
tal development."

2. " Treatment alleviates this condition to some extent, but
it does not, immediately at least, permit the child to gain as he
would if he had not had the disease."

3. " The longer the child has the hookworm disease, the
less will be the improvement mentally when the child is
treated."

4. The last statement Strong considers as " probably the
most important deduction from this whole study."

Such are Strong's conclusions, which corroborate the aver-
age man's casual observation of the apparent effect of hook-
worm disease and, in support of which, all who are interested
in stamping out the disease are eagerly waiting for conclusive
scientific data. But neither Strong nor anyone else has yet
produced such data.

Strong compared the gains made in several of the then most
highly standardized single group intelligence tests after a
period of three and a half months by the following group of
school children:

18 children without hookworm.
9 untreated hookworm children.
27 completely cured hookworm children.
17 incompletely cured hookworm children.

All these children were given the Binet-Simon Test, in
addition to seven other mental tests, calculation test, logical
memory, opposite test, memory span, handwriting, form-board
test, all of which, together with the Binet test were repeated
after an interval of three and a half months. The difference
between chronological age and Binet age for the respective
groups was found to be: non-hookworm children, — 1.0 year;
completely cured hookworm children, — 1.6 years; incom-
pletely cured hookworm children, — 2.2 years; and un-
treated hookworm children, — 1.2 years. According to
the chronological age distribution for the various groups, all
the non-treated hookworm children fall between the ages 9.9-
10.9 years, while all the other groups are rather evenly divided
between the age intervals 9.9-10.9 years and 11-12 years.

3 Effect of Hookworm Disease on the Mental and Physical Develop-
ment of Children. N. Y. The Rockefeller Foundation, 1916.

220 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

Now it has been pointed out that for ages above 10.5 or 10
years, mental age cannot be accurately determined by the old
Binet test, which fact Strong himself records in a footnote
of his study (page 74). Of this shortcoming of the old un-
revised Binet test which was used by Strong, Terman has to
say the following: "The proportion of feeble-mindedness
among adult subjects was greatly overestimated, because sub-
jects who were really of the 12 or 13 years mental level could
only earn a mental age of about 11 years."* He finds fur-
thermore that young subjects get too high a mental age : " In
young subjects the higher grades of mental deficiency were
overlooked, because the scale caused such subjects to test only
a little below normal."

There is a very high probability then that Strong's mental
ages are not the true mental ages and that the non-treated
hookworm group, a comparatively young group, had a mental
age deficiency of considerably over 1.2 years; and that since
all the other groups fall pretty heavily into the 11 to 12 age
group, their mental age deficiency found by Strong is too
great. Indeed, on the basis of the age distribution of the
groups it is highly probable that for increasing order of mental
age deficiency the groups stand : non-hookworm children, com-
pletely cured hookworm children, incompletely cured hook-
worm children and untreated hookworm children.

Strong's data on the social status of the groups help
corroborate this probability, for of the non-hookworm chil-
dren 94% came from the best grade of homes, 0% from
the poorest. Of the completely cured hookworm children
41% came from the best grade of homes, 15% from the
poorest. Of the incompletely cured hookworm children 12%
came from the best grade of homes, 29% from the poorest.
Of the untreated hookworm children 33% came from the best
grade of homes, 56% from the poorest. This is considered
a corroboration of the assumption thatjbetter social ^rrdrpn-
ment on the whole presupposes a higher level of intelligence
in that environment. Considerable data5 are available in sup-
port of the latter assumption.

It so happened that on the whole the gains in the second
trial of the several mental tests (other than Binet) classified
the groups in the order, non-hookworm children, completely
cured hookworm children, incompletely cured hookworm chil-
dren and untreated hookworm children, which is the same

* The Measurement of Intelligence, Lewis M. Terman, p. 3.
5 Lewis M. Terman. The Measurement of Intelligence, pp. 72-73.
Robert M. Yerkes, James W. Bridges, Robert S. Hardwick, A Point
Scale for Measuring Mental Ability.

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 221

order as the very highly probable mental age order of the
groups to start with, as was shown above. // the latter be
true, then the order of gain is but to have been expected, hook-
worm or no hookworm. It is now generally accepted as a
fact that the more intelligent learner learns the more rapidly.
Strong* himself has furnished data elsewhere which he ap-
parently had forgot when interpreting his data on hookworm.
He concludes from an experiment on school children:

" The slope of learning curves of school children based on
simple arithmetical combinations apparently correlates to a
very considerable extent with the general intelligence of the
children." A few years, previously Colvin, practicing- five
normal subjects with five subnormal at canceling A's, found
that " in every case the normal child made greater improve-
ment with less fluctuation than did the subnormal child."7

Likewise the writer found from a card sorting practice on
27 normal school students for 50 minutes a day, with the
practice repeated after 10 days, 11 days and Sl/2 months, re-
spectively, a positive correlation of -(-.48 between maximum
gain and intelligence, when there was no correlation between
initial performances and intelligence. This indicated pretty
clearly that the +.48 correlation showed a very real positive
relationship between intelligence and learning ability. It
should be noted that " intelligence " was determined by having
each student rank all the other students of the class on the
basis of her estimate of each student's intelligence, from which
a combined ranking was computed.8

Assuming, then, that the four groups studied by Strong had
at the beginning of the study mental capacity in the order, —
non-hookworm children, completely cured hookworm children ;
incompletely cured hookworm children, and untreated hook-
worm children; which his difference between chronological
and mental ages (p. 21) indicates when these data are inter-
preted in the light of Terman's Revision of the Binet tests,
that the gain, therefore, for these groups after $l/> months

6 Learning Curve as a Diagnostic Measure of Intelligence, — Psycho-
logical Bulletin, 1917, Vol. 14 pp. 153-154.

7 S. S. Colvin, Notes on Certain Aspects of ' Learning — Psycho-
logical Bulletin, Feb. 15, 1915, Vol 12.

8 Garry C. Myers, Some Variabilities and Correlation in Learning.
Amer. Jr. Psychol. July, 1918, Vol. 29 pp. 316-326.

Since completing this study the writer has demonstrated in a prac-
tical way that learning progress is in proportion to intelligence ratings,
in a school of 1,800 illiterates whom he classified on the basis of intelli-
gence ratings. See Principles, Plans and Purposes of the Recruit
Educational Center, Camp Upton, N. Y., War Department, Washing-
ton, D. C,

222 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

should show exactly that order is just what should be expected,
regardless of hookworm. Apparently, then all Strong's study
has shown is that brighter children learn faster than duller
children.

It must be said of Strong's work that his method of at-
tempting to measure the effect of hookworm on mental de-
velopment in terms of relative learning xgains by hookworm
and non-hookworm children is highly commendable. His
method, if refined by comparing groups of equal mental and
social status, on the basis of their learning rate as measured
by learning curves developed with considerable practice on
specific learning tasks and on large number of children,
should help measure with a good deal of exactitude the influ-
ence of hookworm disease on intelligence.

Strong also studied 11 children — not included in the part
reviewed above ; who on the average were 13.5 years of age,
comparing this group with the 17 incompletely cured hook-
worm children averaging 11.1 years chronologically, both
groups coming " from homes of approximately the same eco-
nomic conditions." The average mental ages for the respec-
tive groups, by Binet-Simon tests, were found to be 9.0 years,
and 8.5 respectively, from which are derived intelligence quo-
tients of .67 and .77. Of course, here again the real differ-
ence is not quite so great if the error of Binet for the older
children is taken into account. Nevertheless, it is obvious
that the older group of 11 children, as pointed out by Strong,
is appreciably lower intellectually, which fact would presup-
pose a lower rate of gain by them, which gain was 1.5% as
against 5.9% for the younger group. " When the two groups
are compared with normal children, we see that the younger
children have gained but 34% of what the healthy children
accomplished, while the older children have gained but 9%."
It should be recalled that this " normal " group had an aver-
age chronological age of 11.1 years and mental age of 10.1
years, giving an intelligence quotient of .91. Certainly, there-
fore, Strong is not justified because the older hookworm chil-
dren with lower intelligence gain less than the younger hook-
worm children of higher intelligence, in concluding : " The
longer the child has the hookworm disease the less will be the
improvement mentally when the child is treated" And not
justified in spite of the fact that he notes, " Our careful study
made of these children (the 13.5 year old children) did not
reveal any other cause for the retardation than hookworm
disease." Here again he has merely shown by his data that
the brighter children learn faster than the duller children,
which is to be expected. Furthermore, it is not clear on what

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 223

ground Strong assumes that the 13.5 year old children with
hookworm have had the disease longer than the 11.1 year old
children.

In 1917 Truman Lee Kelly9 reported a study in which his
results disagree with those of Strong's to the effect that chil-
dren cured of hookworm improve more rapidly in mental
traits after cure than do the normal children.

He too had a small number of cases in the groups compared :

47 children free from malaria and hookworm.

9 children cured of hookworm.

11 children cured of malaria.

11 children cured of malaria and hookworm.

The tests were Starch, Arithmetic, Courtis Arithmetic,
Trabue completion and Thorndike Reading tests, which were
repeated after a six months' interval. Kelly gives no indica-
tion as to the mental status of the group compared at the time
of the first test. Furthermore, since he depended upon the
children's report as to whether or not they had malaria one
is not sure this factor is wholly eliminated from the " f ree-
f rom-malaria and hookworm " group.

If number of cases were the solution of this problem, the
solution should have been near at hand upon the report of
Waite's Study,10 wherein he tested 116 non-infected children,
65 lightly infected children and 159 heavily infected children,
with Goddard's Revision of the Binet-Simon Tests, and the
Porteus Mazes. Waite classified his group in this way :

" 1. The heavily infected cases presenting hookworm ova
in plain smear examinations of stools ;

" 2. The lightly infected cases, presenting hookworm ova
only in smears of centrifuged stools ; and

" 3. The non-infected cases, who showed no ova in four
successive stool examinations, two of plain smears, and two
of centrifuged smears."

Expressed in mental age the retardations for the respective
groups for Binet and Porteus tests were as follows:

9 Effect of Malaria and Hookworm upon Physical and Mental
Development of School Children. Elementary School Journal, Vol. 18,
1917, pp. 43-55.

10 Dr. G. H. Waite, A Study of the Effects of Hookworm Infection
upon the Mental Development of North Queensland School Children.
The Australian Medical Journal Jan. 1919. The original manuscript
dated 1918 was loaned the writer through Major John L. Riley, by
the Library of the Rockefeller Foundation.

224 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

Retarded by Binet...

Non-Infected
3. 9 mo.

Lightly Infected
9. 3 mo.

Heavily Infected
23. 4 mo.

Retarded by Porteus

2. 7 mo.

4 . 7 mo.

16. 00 mo.

Unfortunately Waite computed the average retardation
rather than the median, thereby giving undue weight to the
extremely low cases. Nevertheless, these data show conclu-
sively that the hookworm children studied are on the average
more retarded mentally than the non-hookworm children, and
the heavily infected group is more retarded than the lightly
infected group. But they do not conclusively support the in-
ferences derived therefrom by Waite, wherein he concludes:

1. " The results bring to light clearly two features, namely,
that hookworm infection produces in growing children severely
arrested mental development, and considerable mental slug-
gishness."

2. " The duration of hookworm infection is important, be-
cause the longer the infection persist the greater is the mental
retardation."

3. " The degree of hookworm infection, and therefore the
amount of anemia, definitely influences the amount of re-
tardation of mental development."

4. " The most backward hookworm children invariably give
the history of an early infection."

Considering Waite's first point, there arises the ever-present
problem in interpretation of such data which he seems wholly
to ignore, namely, how is anyone to know from any data which
merely shows the hookworm group inferior in mental rating
whether the inferiority is due to hookworm or whether because
of their inferiority such children were of poor sanitary habits,
thereby more exposed to the infection? In other words, to
what degree does hookworm select the lower level of intelli-
gence? Certainly Waite is not justified by the data he fur-
nishes in making his first conclusion. On the basis of his
data one could infer with just as much logic that " severely
arrested mental development, and considerable mental slug-
gishness " tended to make the children easy victims to hook-
worm.

The validity of Waite's succeeding inferences hangs upon
that of his first one.

In the study of Major Kofoid and Pittenger (see this paper,
p. 211), their comparative data expressed in 10% steps from
the highest to the lowest for hookworm and for non-hookworm

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 225

cases, respectively, indicate an even more pronounced tendency
at convergence toward the lower extreme than does this study.
Here are their data for Alpha:

501 Hookworm

Cases
Aver. Score

1st (Highest) 10%
2d
3d
4th
5th
6th
7th
8th
9th
10th (Lowest)

Median score.

181.0

146.6

118.6

99.0

80,
63,
48,
36.
24.6
1.0

4,792 Non-Hookworm

Cases

Aver. Score
219.0
178.5
150.6
127.7
107.4

89.6
70.7
51.3
31.8
0.0

Although their data were given in weighted scores which
are about twice the unweighted scores in the present study,
thereby rendering the two sets of data not wholly comparable,
yet the general tendency is the same as that shown by our
data. Of course their relative inferiority of the hookworm
group is very much greater than that for our study, they also
found a greater increase of difference from the lowest mental
level to the highest. This is to be expected since their nega-
tive group was selected at random including troops undoubt-
edly from the larger urban populations and other non-infected
or lightly infected areas, whereas each of our hookworm cases
was paired with a non-hookworm case from the same local
territory. They also found in accord with our data, that the
relative inferiority of the hookworm troops was much greater
for whites than for colored and greater also for Alpha troops
than for Beta troops.

Interpreting these data Kofoid and Tucker say : " In liter-
ate blacks the difference is less, as might be expected, because
of a relative racial immunity." They conclude:

"The interpretation of the data is that men infected with
hookworm belong to all classes of mental rating, but not
wholly proportionately in all groups to those without the in-
fection, the men with the low scores showing a greater relative
deficiency as the score falls. It also appears that the infected
men are pushed en masse below the normal by infection by
hookworm, or by other factors correlated therewith in a vicious

226 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

circle, but that the men with lower scores have a greater
momentum and move relatively farther from the norm than
do infected men with higher scores. This suggests the con-
clusion that men with hookworm include a greater proportion
of subnormal types than men without and that subnormal men
tend to acquire the disease more rapidly than men of higher
categories. It is also possible that the intensity of individual
infections may be heavier in the groups of lower intelligence,
due to the relation of intelligence to sanitation, which in turn
modifies the chances of infection and thus the ultimate in-
tensity thereof."

As for the negroes, they logically fit into the lower order,
consequently showing, as do the lower whites, a comparatively
small inferiority for the hookworm group. Then the question
may arise why is not the convergence consistent from the
highest to the lowest extremes of intelligence instead of a
rather sudden convergence toward the lower extreme? The
answer may be that with the exception of the lowest 30 or
40% of the intelligence scale of a community, there is hardly
to be expected a great improvement in sanitary habits near
the more intellectual extreme. Certainly the degree of rise
in standards is hardly proportional all the way upward. On
the other hand, just why should the very upper extreme of
the hookworm and non-hookworm groups be actually closer
than the mean where there is a slight bulging in the space
between the curves? Undoubtedly this is due to a failure of
Alpha to differentiate as widely in intelligence scores for the
upper extremes of intelligence. In the numerous studies of
Alpha by the Division of Psychology it was invariably found
that, on the basis of a man's military value, Alpha did not
clearly discriminate between the upper and lower A grade
man, nor even always between the A and B man.11 Further-
more when given to college men, the discrimination in terms
of intelligence rating is certainly not very pronounced.12

On the other hand, the convergence at the lower extreme
may mean that hookworm disease in some peculiar way does
not affect persons of very low mentality or even those of very
high mentality or that the effect is so small as to be registered
but slightly or not at all by an intelligence test. In this event
the difference between the hookworm and non-hookworm
groups would undoubtedly be due to hookworm infection.

If two groups, say from northern states where hookworm

11 Psychological Examining in the United States Army, Part III.
P. published by the National Academy of Sciences, Vol. XV.

12 School and Society, Vol. X, No. 250, pp. 437-440.

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 227

is practically absent, whose median mental status were known
to be quite disparate, were compared on the basis of 10%
steps, comparison with such data would help clear up the
difficulty in interpreting the lack of complete parallelism be-
tween the 10% steps of the hookworm and non-hookworm
groups of this study.

In reading the data of Table I care should be exercised in
interpreting per cents of difference in respect to all mental
ratings since a year's difference between mental ages 7 and 8
years cannot mean the same as a year's difference between 11
and 12 years, nor in terms of per cent, since even in spite of
intelligence scales there is no certainty that the increments in
terms of mental age or even in terms of Alpha or Beta scores
are constant. Let such per cents merely be read as ratios to
the higher values compared and no more.

RELATION OF SCHOOLING TO HOOKWORM
When the amount of schooling reported is considered the
data are no more conclusive in what they mean, for very high
correlations were found between the army group tests and
amount of schooling.13

Just what the high correlation between intelligence rating
and schooling means is not certain, i. e., one does not know
how much schooling offsets ability to score in the army tests
nor to what degree the amount of native ability determined how
long school had been attended. In this study exactly the same
problem arises: Did hookworm disease decrease the amount
of school attendance or was the kind of children whose school
attendance was limited, of those children who most readily
fell heir to the disease ? Maybe such children were those less
likely to have shoes and not having so much schooling and
not coming perhaps from so good a social environment would
not have developed so good sanitary habits.

RELATION OF AGE TO HOOKWORM

Since all the troops from among whom the cases selected
were of practically the same age distribution, it is quite sig-
nificant that the hookworm troops are from .7 to .9 of a year
younger than the non-hookworm troops.

According to the report of the Porto Rico Commission,14
there was found the following age distinction :

13 Psychological Examining in the Army, Part III. National Acad-
emy of Sciences, Vol. XV.

14 Ashford and Jgaravidez, Uncinariasis in Porto Rico, Sen. Dec.
808, Washington, D. C, 1911.

228 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

Age Cases

Under 10 years 15,622

10 to 20 years 50,924

21 to 30 years 36,589

31 to 40 years 18,254

41 to 50 years S,/96

51 to 60 years 3,841

Over 60 years 1,413

Although the cases examined were not exactly a random
sampling, these figures doubtless represent the approximate
age distribution, and are slightly suggestive of a corroboration
of our findings.

A study reported by Major Siler and Major Cole in 1917
is more to the point.15

Of the First Mississippi Infantry 32% were infected, and
the First Alabama Cavalry, 54%. The age distribution for
the highest 10 years were as follows:

First Mississippi First Alabama

Years % %

16-17 1 2

18-19 21 42

20-21 32 24

22-23 20 12

23-24 11 7

Obviously the First Alabama Infantry, a greater per cent
of whose troops have hookworm, are younger than those of
the First Mississippi. In the light of these data the writers
inferred (p. 95) :

" In our opinion there is but little or no doubt that age
accounts to some extent for the greater prevalence of both
hookworm disease and measles in the First Alabama Cavalry."

From this data and from the present study (see plate 25-28)
it seems that the younger troops are more susceptible to hook-
worm disease than the older troops. Certainly more data is
desirable as to the exact distribution of hookworm cases on
the basis of total population at various ages and in various
racial levels. However, it is also probable, since hookworm
infection is limited in its length of life within a given indi-
vidual, that many of the older men, who, in the hookworm

15 Major J. F. Siler, M. C. and Major C. L. Cole, M. C, The
Prevalence of Hookworm Disease in the Fourth Texas Infantry. Fir^t
Mississippi Infantry, and First Alabama Cavalry Regiment. The
Military Surgeon, 1917, Vol. 41, pp. 77-99.

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 229

survey fell into the non-hookworm group, had once been hook-
worm cases but had become free from hookworm at some
time previous to the hookworm examination.

CONCLUSION

The average person of a given community, who is infected
with hookworm, is inferior mentally to the average person,
of the same community, who is not infected with hookworm.
In support of this statement conclusive data have been pre-
sented.

But there are yet no data available to show conclusively
whether this mental inferiority is due to hookworm disease
or whether hookworm disease is due to mental inferiority.

A fact not calling for evidence here is that hookworm dis-
ease is a social and economic burden upon tropical and sub-
tropical countries. Now if it does not in and of itself lower
the mentality of its victim, there is ample evidence that it will,
as will any other disease, diminish his educational opportuni-
ties, and thereby his efficiency^as a social unit.

Since because of the very nature of the transmission of
hookworm disease, wholly dependent upon remedial sanitary
habits and conditions, doubtless it is true that just because of
lower intelligence or lower mental training the victim more
readily falls heir to the disease; hence the obvious remedy is
more education to the educable, a far stricter governmental
guardianship over the non-educable and over those educable
to but a slight degree, and an aggressive general governmental
control over the social habits and sanitary conditions of the
community. In matters of education, mere ability to master
the three R's should not be the goal alone, but through this
median specific instruction in matters of simple rules of health
and sanitation is of imperative necessity.

This emphasis upon specific hygiene instructions is obviously
none the less imperative if it is discovered that the hookworm
infection is primarily the cause of the relatively lower mental
status of its victims.

So, then, whether hookworm disease is the cause of mental
inferiority or the result, or both, the fact that those infected
with the disease are, as a rule, distinctly inferior to those free
from the disease, as shown conclusively by this study, is an
inexorable challenge to the federal, state and local leaders in
matters of" health education and general economic and social
betterment, to use every possible means to rid the country of
this pesf

230 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

SUGGESTIONS FOR FURTHER RESEARCH

The question remaining to be answered is, Does hookworm
disease directly depreciate the mental status of its victims?

It has been seen that Strong's method of comparing gains
in learning by the cured-of-hookworm children with hookworm
children, while on the right track apparently, was fallacious
in that it ignored the fact that ability to learn correlates highly
with general intelligence; and, that as subsequent studies and
as his own data indicate, almost any randomly selected hook-
worm group will be found to be of lower mental status than
a non-hookworm group similarly selected.

It is necessary, therefore, in order to make a fair compari-
son, to be sure that the groups compared are at the time of the
first learning test of practically the same mental ability with
practically equal distribution of abilities in the groups com-
pared.

Furthermore, the social status of the compared groups
should be equal. A study of infected and non-infected chil-
dren of the same family (especially of twins) would meet
both conditions fairly well. That the social status be equal-
ized in such cases would be obvious and that twins show far
closer mental resemblance than siblings and siblings far closer
resemblance than randomly selected children, has been well
established by Galton,1*5 Thorndike,17 Rusk,18 Cobb,19 Starch,20
and others.

In the absence of either class of subjects to be studied, the
group intelligence tests available would enable the selection
of groups of practically the same mental abilities.

If, then, of those two groups, the hookworm group learned
appreciably more slowly than the non-hookworm group or
cured-of-hookworm group, the obvious inference would be
pretty safe.

The learning task should extend over a period of several
weeks or months including numerous practices, thereby offer-
ing opportunities to measure mental endurance and attitude
toward a task whose novelty has worn off and wherein the
stimulation of a stranger as examiner has ceased to function
as a stimulation.

16 Sir Francis Galton, History of Twins, p. 170 f of the reprint in
Everyman's Library.

17 E. L. Thorndike, Measurement of Twins, Archives of Philosophy,
Psychology and Scientific Methods No. 1, 1905.

18 Elizabeth Rusk, Mental Resemblance among Siblings. Teachers
College, 1908, Masters Thesis, (unpublished)

19 Margaret V. Cobb, A Preliminary Study of the Inheritance of
Arithmetical Abilities. Journal of Educational Psychology 1917,
Vol. 8 —pp. 1-20.

20 Daniel Starch, The Similarity of Brothers and Sisters in Mental
Traits. Psychological Review 1917, Vol. 24, pp. 235-238.

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 231

Assuming that the infection does not depreciate the mental
organism structurally, even then in terms of attitude there
would remain the question of functional depreciation. It is
desirable then to devise a test to measure the attitude of the
learner toward his task. Admittedly this is difficult of meas-
ure, yet, the difference in the learning gains by two groups
who on setting out upon the task are of equal intelligence,
should be a fairly good measure.

Suppose, for example, two such groups are tested at a given
time and after, say six months, are retested, and show the
same gains. Even then their learning abilities under normal
procedure may not be proved equal ; for if on the other hand
the test had been repeated 75 times on as many days or if the
children had been at regular school work with their teacher,
the one group may have lagged in interest far more than the
other.

Certainly the factor of attitude and of stimulation under
peculiar conditions have not been carefully taken into account
in some of the widely read experiments whose data seem to
show, for example, that rebreathed but circulated air is quite
as good for the child's mental function as freshened air or
that there is no such thing as mental fatigue or that certain
drugs have little or no impairing effect on mental functioning,
when they offer no such conclusive proof at all.

Professor James, somewhere, has pointed out that few per-
sons work up to the limit of their capacity; J. J. B. Morgan,21
that subjects lifting weight not visible, by means of pulleys,
tend to lift with the same speed, within certain limits, regard-
less of the intensities of the weights. It has been shown22
that learners continue to improve under intensive practice to
a very remarkable degree under stimulation of rivalry. In
another study23 the writer has shown that by learning against
time, the ordinary learning record can be improved to sur-
prising degree.

A learning test, then, over a long period of time, with many
practices, on members of a large hookworm group compared
with similar data on a large non-hookworm group, both of
practically the same mental and social status at the beginning
of the test, with the diagnosis and cure of the disease under
control, should yield a definite answer to the question of the
mental influence of hook worm disease.

21 J. J. B. Morgan, The Overcoming of Distraction and other Re-
sistances. Archives of Psychology #35, Science Press.

22 Garry C. Myers, Hazel Coburn, Helen Collins, School and Society
1918, Vol. 8 pp. 597-600.

23 Garry C. Myers, Learning Against Time. Jr. Ed. Psychology,
February 1915, pp. 115-116.

232 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

PLATE 1

PLATE 2

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 233

PLATE 3

PLATE 4

234 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

PLATE 5

PLATE 6

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 235

PLATE 7

PLATE 8

236 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

PLATE 9

PLATE 10

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 237

PLATE 11

PLATE 13

23$ INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

PLATE 13

PLATE 14

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 239

PLATE 15

PLATE 16

240 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

PLATE 17

SUCCESSIVE TEN PER CENT GROUPS

Highest 2nd 3rd 4th 5th 6th 7th 8th

10% 10% 10% 10% 10% 10%

9th Lowest

10% 10% 10% 10%

PLATE 18

SUCCESSIVE TEN PER CENT GROUPS

Highest 2nd 3rd 4th 5th 6th 7th 8th 9th Lowest
10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM 24 1

PLATE 19

SUCCESSIVE TEN PER CENT GROUPS

Highest 2nd 3rd 4th 5th 6th 7th 8th 9th Lowest

10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

COLORED TROOPS

l\ HOOKWO"

9 \ NON-HOOt

- 162 CASES
ZQ2 CASES

PLATE 20

SUCCESSIVE TEN PER CENT GROUPS

Highest 2nd 3rd 4th Sth 6th 7th 8th 9th Lowest
10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

242 INTELLIGENCE OF TROOPS INFECTED WITH HOOKWORM

COLORED TROOPS

BETA

HOOKWORM —

NON-HOOKWORM

290 CASES
-3O6CASES

PLATE 21

SUCCESSIVE TEN PER CENT GROUPS

Highest 2nd 3rd 4th 5th 6th 7th 8th 9th Lowest
10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

COM MON SCALE

HOOKWORM 6I2CASES

NON-HOOKWORM 612 CASES

PLATE 22

SUCCESSIVE TEN PER CENT GROUPS

Highest 2nd 3rd 4th 5th 6th 7th 8th 9th Lowest
10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

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