The Relation Between Intelligence and Age of Walking in Normal and Feebleminded Children
- Author:
Miles Murphy
University of Pennsylvania It is generally recognized that feebleminded children are retarded in beginning to walk. Among the first references to this retardation is a report on the age of walking in 111 cases of mentally deficient children given by Ireland.1 Of these children Ireland says, ‘’The average time before the child began to walk was 2yz years… . Only five cases were stated to have begun to walk at one year.” The first statistical study comparing the age of walking in normal and feebleminded subjects was made by Mead.2 This investigator compared the age of first walking in 144 cases of “schoolable” feebleminded children in a State School with the age of first walking in 50 cases of “bright, normal” children. Mead found that for his feebleminded group the mean age of walking was 25.08 months, the median was 21.06 months with a semi-interquartile range of 7.56. For his normal group Mead found the mean age of walking to be 13.88 months. The median was 13.54 months, with a semi-interquartile range of 1.06.
Additional information, varying from the results of empirical observation to the findings of statistical study, is available on the age at which normal children begin to walk. Holt and Howland 3 state,’’ The first attempts at walking are commonly seen in the 12th or 13th month. The average age at which children walk freely has been, in our experience, the 14th or 15th month. Quite wide variations are seen in healthy children. … We have known infants to walk at 10 months and many others not until 17 or 18 months, although showing no sign of disease.” Demoor 4 says, “One can 1 Ireland, W. W. The mental affections of children, Philadelphia, P. Blakiston’s Sons, 2d ed., 1900, p. 328.
2 Mead, C. D. The age of walking and talking in relation to general intelligence. Ped. Sem., 1913, 20, 460-484. 3 Holt, L. E., and Howland, J. The diseases of infancy and childhood New York, D. Appleton & Co., 8th ed., 1925. 4 Demoor, J. Die anormalen Kinder und ihre erzieliliche Behandlung in Haus und Schule, Altenburg, Oskar Bonde, 1901.
not say at what age children should begin to walk. Weight, physique and sex have a great influence on the time. Nevertheless, it can be asserted that a normal child should begin to walk between the 12th and 18th month.” Gesell,5 in his system of standards for pre-school children says, “The ability to walk alone earns a credit of A at 12 months, and a credit of C at 18 months.” An A rating represents a frequency of from 20 per cent to 49 per cent; a C rating, a frequency of from 85 per cent to 100 per cent. In France Variot6 found that 67 per cent of children walk from 12 months to 15 months inclusive. In his study of gifted children Terman7 says, 11 Comparison shows that our gifted children walked about one month earlier… . than Mead’s normal children.” Smith et al.8 have made an extensive study combining the work of Mead, Terman and an unpublished study of 109 cases in Iowa, with 725 children representing different racial groups in Hawaii. The mean age of walking in the Hawaiian group was 13.25 months; the median, 13.71 months. This study reveals no evidence for any significant racial difference although the authors find that, “as we proceed from colder to warmer climates, we find the children walking at progressively younger ages.” In ten different groups included in their study the median age for walking for girls is lower than that for boys in every group. The authors conclude that there is a significant sex difference, but that it is difficult to estimate its amount. They also find that low intellectual and social status is accompanied by a significant rise in the age of walking, but that high intellectual and social level do not cause a correspondingly significant lowering. “We find a few reports on the relation between degree of defect and age of walking among the feebleminded, or as it might be expressed between intelligence and age of walking. Mead, finding his mean age to be five months less than that given by Ireland, attributed the difference to the fact that his own group consisted of “schoolable” cases while Ireland’s group was made up of 5 Gesell, A. The mental growth of the pre-school child, New York, The Macmillan Co., 1926, p. 75.
e Yariot, G. Sur les facteurs normaux et morbides qui peuvent avancer ou retarder le debut de la marche bipede chez les jeunes enfants, Bull. Sf Mem. Soc. Med., 1927, 51, 353-361.
7 Terman, L. M. Genetic studies of genius, Vol. I, Mental and physical traits of a thousand gifted children, Stanford University Press, 1925, p. 187. s Smith, M. E., Lecker, G., Dunlap, J. W., & Cureton, E. E. The effects of race, sex, and environment on the age at which children walk, J. Genet. Psycliol., 1930, 38, 489-498.
“idiots.” It is quite probable that Mead’s group was superior to Ireland’s, but the designation “idiots” should not be taken too seriously since the term idiot has been used in a less specific sense, particularly at the time when Ireland wrote. Lapage,9 reporting on the age of walking in 155 feebleminded children, divides the group on the basis of teachers’ estimates of mental capacity into Good, Medium and Bad. The mean age of walking for these groups is respectively 1.5 years, 1.8 years, and 2.2 years.
There are likewise a few reports on the relation between intelligence and age of walking in normal children. Abt, Adler and Bartelme10 determined the correlation between I.Q. and age of walking in 1000 cases, 500 boys and 500 girls. They found a coefficient of correlation (Pearson) of ? .36, ? .03 for the boys and ? .37, ? .03 for the girls. This result they consider to be significant. The group must have been decidedly inferior since the average I.Q. was 80 for the boys and 82 for the girls; no case, however, suffered from gross mental deficiency. Aoki11 reports a correlation of .53 between scholarship and age of walking, and a correlation of .19 between age of walking and a group of tests including tapping, memory and cancellation. Presumably the data were so expressed that early walking received a high score or rating. Unfortunately we have had access not to the original, but only to a brief abstract of Aoki’s work.
It is quite evident from the information now available that feebleminded children, when compared as a group with normal children, are strikingly retarded in beginning to walk. There is also a suggestion that age of walking and intelligence are related within both groups. It is needless to point out that this fact is of interest to the clinical psychologist. The present study is an attempt to study the age of walking in a feebleminded and in a normal group from the same clinic population, and to investigate the relation between intelligence and age of walking in each group. The study is based first upon the records of all children examined in the Psychological Clinic of the University of Pennsylvania from January 1, 1926 to June 30, 1932 whom the psychologo Lapage, C. P. Feeblemindedness in children of school age, Manchester, at the University Press, 1911, p. 80.
10 Abt, I. A., Adler, H. M., & Bartelme, P. The relationship between the onset of speech and intelligence, J. Amer. Med. Assoc., 1929, 93, 1351-1355. 11 Aoki, S. Significance of first walking in child’s development, Shiuri Kenkyu (Psychological Studies), Vol. 21, No. 1, Abstract in Psycliol. Abs 1927, 1, 97.
ical examiners diagnosed as feebleminded, a total of 408 cases. A case history is obtained for each individual examined in the Clinic, and one item of information included in this history is the age of first walking. This information is secured from the responsible person or persons bringing the child for examination, usually the mother. In 30 cases it was not possible to learn the age at which the child first walked. Most of these cases were children brought to the Clinic by social agencies or institutions into whose hands the child had come under circumstances which made it impossible to obtain the desired information. In 13 cases the children were not able to walk at the time of examination, and in 3 cases the mother could remember only that the child had walked at the “average” age. When these 46 cases, slightly more than 11 per cent of the original number, are discarded there remain 362 cases in which we have a report of the age at which the child first walked. Reference to a previous study showed that during this period the proportion of normal to feebleminded children among cases examined in the Clinic was approximately five to one. It was decided therefore to take for the normal group every fifth case examined during this period and diagnosed as of normal mentality. For this group 350 cases were obtained. The mean I.Q. of this group was 106. When feebleminded children are eliminated, as is naturally the case with this group, the Clinic population seems to be slightly superior. A tabulation of the data just as found in the records is shown in Table I.
No doubt it will be objected at once that the data upon which our investigation is based are open to error with the feebleminded and normal cases alike. First, the situation in question, that is, the age at which the child first walked, is separated from the time of report by an interval of considerable length. The median chronological age of these cases at the time of examination was just under nine years. Again, the report depends upon memory, and not in most cases upon anything in the nature of a written record. Finally, the observation and subsequent reports are made by untrained observers who may not always have in mind a standard performance when giving their report. By age of first walking we mean the age at which the child first took steps alone, but we can not be sure that this can be held constant in the minds of all our observers.
Unfortunately, these objections can not be entirely overcome. However, we can say that for any large number of cases of feebleminded children the report of parents is the only information availINTELLIGENCE AND AGE OF WALKING 191
Table I Age of Walking in Feebleminded and Normal Groups Age in Months Feebleminded Normal 7 0 1 8 1 1 9 8 15 10 2 16 1 1 9 33 1 2 18 72 1 3 13 37 1 4 19 56 1 5 16 24 1 6 13 16 1 7 7 11 1 8 42 36 1 9 1 1 2 0 12 5 2 1 8 3 2 2 11 4 2 3 5 0 2 4 47 12 2 5 3 0 2 6 2 0 2 7 3 1 2 8 1 0 29 2 0 30 33 3 3 3 1 0 3 4 2 0 36 47 2 38 1 0 42 11 0 48 13 0 54 2 0 60 4 0 66 1 0 72 4 1 Total 362 350
able on the age of first walking. This important developmental step usually takes place in the home with parents the only observers. Even if it were possible to determine by much more accurate observation the age at which a group of unselected children begin to walk, the percentage of feebleminded children is so small as to make the resulting group extremely limited.
As far as our own data with respect to feebleminded children are concerned we may say that if it is true, as the antecedent evidence indicates, that feebleminded children are retarded in beginning to walk, then it is likely that this retardation will impress itself upon the attention of parents and lead to more careful observation than would be the case with a normal child. It might be added, purely as a psychological conjecture, that any inaccuracy in the report of parents is likely to be in the direction of de192 THE PSYCHOLOGICAL CLINIC creasing rather than increasing the age reported for first walking. Differences discovered between the normal and feebleminded thus would tend to be conservative.
Inspection of the results for the feebleminded group as shown in Table I, reveals still another difficulty to be encountered in our data. It will be noticed that the distribution is bimodal, if not multimodal. This situation can be explained in part by errors in observation. The unit in which a child’s age is expressed both in a general sense, and with respect to a particular problem such as the one with which we are dealing here, changes as the child grows older. In early life the infant’s age is expressed in weeks or months, later in years and fractions of a year. If we ask a mother how old her child is, and he is less than two years old, the answer will probably be in months. If he is between two and three years old, the answer may be in months but is more likely to be in years and a fraction of a year. If he is past three years of age, the answer is almost certain not to be in months. This fact of common observation naturally influences the report of parents when they attempt to recall the age at which a child first walked. It will be seen upon examining the results for the feebleminded group as shown in Table I that after 18 months there is a tendency for the cases to accumulate at the half-year periods: 42 cases at 18 months, 47 cases at 2 years, 33 cases at 2^ years, 47 cases at 3 years, etc. After 18 months 76 per cent of the cases are reported to walk at either year or half-year intervals.
The fact that the results for normal children differ so much from the results for feebleminded children raises a problem in the statistical treatment of the data. The nature of the results for the feebleminded group calls for some grouping of the data into step intervals because there is no reason to believe that age of walking among feebleminded children is disturbed in any such discontinuous fashion as these figures would indicate. Consequently in the treatment of the results, the data for the feebleminded group are grouped into intervals of six months. A similar process is, of course, impossible with the normal children since more than 60 per cent of the cases begin to walk between the ages of 12 and 17 months. The results for the normal group are treated therefore in intervals of one month.
Of the feebleminded group 63.2 per cent were boys, and 36.8 per cent were girls; of the normal group, 68.9 per cent were boys, and 31.1 per cent were girls. It is characteristic of the clinic population to show more boys than girls, although it will be noticed that the preponderance of boys is not quite so great among the feebleminded. Statistical analysis revealed no significant difference between boys and girls. In both groups the mean age of walking was slightly lower for boys than for girls. This result is of some interest in the light of the findings of Smith et al., but since the difference was not even equal to its sigma in either case, we can not consider it of any significance. The results of our study are consequently presented in this paper without differentiating between the two sexes.
Still another problem arises in connection with the treatment of the data. Smith et al. in treating their results discarded all cases which did not walk until after the age of 21 months as being atypical, probably feebleminded or physically defective. In our normal group the possibility of feeblemindedness is ruled out by diagnosis, although physical defect is not. As a matter of fact it seems likely that in a group of clinic cases atypical cases of all kinds are more frequent than in the general population. This is a defect to which all statistical studies based on clinic cases are subject and consequently we do not propose to make any general conclusions on the basis of our findings. Since we are comparing a normal group with a feebleminded group we can not arbitrarily eliminate certain cases from our normal group, at the same time we would like, as far as we can, to make our normal group comparable with the groups involved in previous study. We therefore give our results both with the atypical cases walking at more than 22 months eliminated, and with those cases included.
In comparing the feebleminded and normal we find the following results: Mean Median S.D. a Mean Skewness Feebleminded Normal (all cases) Normal (cases above 22 months discarded) 26.65 14.99 14.16 23.70 14.00 13.74 12.78 5.10 2.84 .67 .27 .16 .69 .58 .44 47.95 34.02 20.06
It is scarcely necessary to point out that the difference between the feebleminded and normal groups is a reliable one. “When all cases are included the difference is 16 times its sigma; when the atypical cases are eliminated the difference is 18 times its sigma Of the feebleminded group 87.6 per cent walk at an age later than the median age for the normal group.
It should be noted that both distributions are skewed positively, and to that extent our findings are open to possible error. It will be noticed that our results do not differ greatly from those of Mead who found a mean of 25.08 for the feebleminded and a mean of 13.88 for the normal. The slight variation may be due to a difference in the sample since Mead used “schoolable” feebleminded, and “bright, normal” children. In both Mead’s groups the median is less than the mean, and it would seem that the distribution of age of walking tends to be positively skewed.
A further problem which concerns us is the relation between intelligence and age of walking in the normal group. An I.Q. on the Stanford Revision of the Binet-Simon Tests is available for all our normal children. The coefficient of correlation between I.Q. and age of walking, when computed by the Pearson Product Moment method proves to be ? .19 ? .035. In calculating this coefficient the 7 cases not walking at two years of age were discarded as highly atypical. Reference to Fisher’s tables12 shows the probability that this result may be due to chance to be less than 1 in 100. It seems safe to say that there is a negative correlation between I.Q. and age of walking, but we can not be sure of the amount. Testing for linearity shows the correlation to be probably, but not certainly, curvilinear, so that our result may underestimate the true relation. We may proceed further in the study of .the relation between age of walking and I.Q. If we separate our normal group into three subgroups on the basis of age of walking, we find that 66 cases or approximately 20 per cent are reported to walk before one year of age; 216 cases, approximately 60 per cent, are reported to walk at from 12 to 17 months inclusive; finally, 68 cases, approximately 20 per cent, are reported to walk at 18 months or later. These groups will be referred to as Group I, Group II and Group III respectively. Our normal group is thus divided roughly with respect to age of walking into an upper quintile, a lower quintile, and a median modal group of 60 per cent. The atypical cases referred to above are included in Group III. The mean I.Q. is 106.1 for Group I, 109.7 for Group II, and 93.2 for Group III. The significance of these differences may be analyzed as follows: 12 Fisher, E. A. Statistical methods for research workers, Edinburgh, Oliver & Boyd, 4th ed., 1932.
Groups Difference er difference Diff. a diff. I and II… II and III. I and III.. 3.6 16.5 12.9 2.9 2.5 3.3 1.2 6.6 3.9
Group III is significantly inferior to Groups I and II. This is also shown by the fact that 77 per cent of Group II and 71 per cent of Group I have I.Q.’s above the median of Group III. Group II is superior to Group I, but this appears to be a chance variation. While these results are not to be considered as conclusive, they suggest that children accelerated in walking are not a relatively superior group, but that children retarded in walking are significantly inferior.
The relation between intelligence and age of walking can be studied now in the feebleminded group. In 254 cases of this group we have a Stanford-Binet I.Q. In the remaining cases no I.Q was secured. The coefficient of correlation between age of walking and I.Q. is ? .23, ? .04. This coefficient is very nearly the same as that found for the normal group, and the remarks made concerning that coefficient are applicable here.
If we divide the feebleminded group into sub-groups as we did the normal group, we find 84 cases walking before 18 months, 118 cases walking at ages from 18 to 35 months inclusive, and 53 cases walking at 3 years or over. We shall refer to these groups as Group A, Group B and Group C respectively. The mean I.Q. of Group A is 58.81; of Group B, 54.88; and of Group C, 48.8. The significance of these differences may be analyzed as follows:
Groups Difference a difference Diff. or diff. A and B. B and C. A and C. 3.93 6.08 9.93 2.2 2.41 2.53 1.8 2.5 3.9
Group C is certainly inferior to Group A and probably inferior to Group B; we are not justified, however, in saying that the difference between Groups A and B is due to factors other than chance. Another means of studying the relationship between the degree of mental defect and age of walking is available. In 283 cases the diagnosis carried with it an estimate of the degree of mental deficiency on the Barr Classification. In 79 cases no statement was made of the degree of deficiency. Of the remaining cases 31 were diagnosed as high grade imbeciles, 59 as middle grade imbeciles, 120 as low grade imbeciles, 65 as idio-imbeciles, and 8 as idiots. Before comparing these groups, we must examine the 79 cases in which no reference was made to the degree of defect. The average age of walking for the feebleminded group as a whole is 26.65 months. For these 79 cases the age is 27.39 months. The difference is only .58 of its standard error, when analyzed by Pearson’s method for the comparison of the sub-group with the total group of which it is a part. We can say then that the cases in which the degree of defect is estimated, do not constitute a selected group from the view-point of age of walking.
If we return then to the groups mentioned we find the following results. The mean age of walking for the high grade imbeciles is 21.58 months; for the middle grade imbeciles, 24.05 months; for the low grade imbeciles, 25.35 months; for the idio-imbeciles, 32.17 months. The average age of walking, it is seen, increases as we pass from the milder to the more severe degrees of defect. The differences between these classes may be analyzed as follows:
Classes Difference a difference Diff. a diff. HGI and MGI. HGI and LGI. HGI and II… MGI and LGI. MGI and II.. . LGI and II…. 2.47 3.77 10.59 1.30 8.12 6.82 2.11 2.04 2.45 1.74 2.21 2.14 1.17 1.85 4.32 .75 3.68 3.14
The idio-imbecile group is significantly inferior to all other groups, it would appear from these results. The differences between the other groups, however, are not statistically reliable. Additional significance, however, attaches to these results since there is in every instance an increase in age of walking as we go from a less pronounced to a more pronounced degree of defect.
Before leaving this problem attention should be called to the fact that while, in comparing sub-groups, the differences are frequently not statistically reliable, nevertheless, in all instances except one (sub-groups I and II in the normal group) we find the subgroups differentiated in that direction which indicates retardaINTELLIGENCE AND AGE OF WALKING 197 tion in walking to be a sign of inferior intelligence. “We believe there is shown in these findings a cumulative significance not entirely revealed by the detailed comparisons. In the total group of feebleminded there were 45 cases which were diagnosed as belonging to the Mongolian type of mental deficiency. This type of mental abnormality has been the subject of much study, and is of particular interest because it is accompanied by such definite and characteristic signs. The Mongolian group was treated separately. The mean age of walking in the Mongolian group is 34.6 months, higher than any of the groups mentioned before, and nearly 8 months higher than the mean for the entire group of feebleminded. When this mean is compared with the mean for the entire group using the formula of Pearson already referred to, the difference proves to be 4.4 times its sigma. This difference may be accounted for partly by the fact that the Mongolians belong to the lower degrees of mental deficiency, and also by the lack of muscular tone and coordination characteristic of Mongolianism. Our results seem to agree with those of other investigators. Ordahl13 found the median age of walking in 159 Mongolians to be 37 months. Ivuenzel14 found Mongolians walking about 8 months later than a control group of other feebleminded children.
Summary
The mean age of walking in 362 feebleminded children was 26.65 months; in 350 normal children, 14.99 months. The correlation between age of walking and I.Q. was ? .19, ? .035 for the normal group, and ? .23, =t .04 in 254 feebleminded cases for whom an I.Q. was available. Normal children not walking before 18 months are significantly inferior in I.Q. to those walking before that age. Idio-imbeciles are significantly later in walking than the less pronounced degrees of mental deficiency.
is Ordahl, G. Birth rank of Mongolians, J. Hered., 1927, 18, 429-431. i-i Kuenzel, M. W. A survey of Mongolian traits, Tr. School Bull., 1929, 26, 49-58. 13
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