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Boas, Franz. Race, Language and Culture – T06


Influence of Heredity and Environment
upon Growth 11

We have seen that among the individuals composing a population
considerable differences in the rate and ultimate result of
development are found and that these depend largely on external conditions
under which growth and development take place. The question
arises whether it is possible to separate the variability caused by environment
from hereditary conditions.

In an attempt to answer this question we have to investigate the relations
between the bodily forms of parents and children and those between
members of fraternities. The method by which these relations
can be investigated may be explained by the example of the head index.

If in a given population fathers with unusually rounded heads are
compared with their children, it is found that their children have also
the tendency to have rounded heads, but less so than the selected group
of fathers. For instance, if the average head index of a population is
80% and a sufficiently large number of fathers with an index of 86%
is picked out, the average head index of the children of this group will
be 82%, about one-third 22 of the deviation of the fathers from the norm
of the selected group of fathers. In this way all the body parts may be
investigated and a relation similar to the one just mentioned may be
established. In populations which have lived for a long time in the
same place and whose families have intermarried the “similarity” of
fathers and children, as here defined, is about one-third. The same value
has been found for the similarity of mothers and their children. The
similarity among brothers and sisters is much greater. In a population
like the one just referred to a group of men may be selected whose headform
82is characterized by the index 86%. Then the average index of
their brothers will be 83%, in other words the difference between the
norm and the average of the brothers of the selected group is about one-half
of the difference between the norm and the value of the selected
group. We may say that the similarity of the brothers is about one-half.

Observations regarding the similarities between parents and children
and those between brothers and sisters are similar to those mentioned
here: about one-third and one-half. Often the values are a little different.
Much larger values have been found for twins, particularly
for identical twins — that is, those developed from a single ovum. For the
present the values given here may be considered as the norms for
European populations.

In parts of the body which in the course of growth undergo marked
individual changes, such similarities may be considered in the following
way: Besides the hereditary similarities which are determined by the
values just given each individual develops independently of all the
others, with varying tempo and intensity, according to more or less
favorable conditions of life. If these causes are of such a character that
they are expressed in the bodily form of the adult, there must be, besides
the hereditary causes, other accidental ones which are different for the
different individuals. In this case a weakening of the similarities may
be expected. Therefore, if the observed similarities are so arranged that
the similarities of those forms which are permanently established shortly
after birth are compared with those which take their permanent forms
later in life, it may be expected that the latter group will show lower
degrees of similarity.

I have carried through this inquiry for the statures of East European
Jews measured in New York. The results show a considerable decrease
for the degree of similarity as compared to that of the measurements of
the head. The similarity between parents and children for length and
width of head, which are established early in life, obtained from more
than 2,300 observations is about .36, while that for stature referring
to parents and their adult children is only .21. The similarity of siblings
(brothers and sisters) in regard to stature is .33 as compared to .56, the
similarity for head measures. We may conclude from this that the
reduction of variability is essentially due to external differences under
which the individuals develop. 1383

If it is assumed that the hereditary part of the similarity is the same
for all parts of the body — an assumption that needs proof — it is possible
to determine the part of the variability to be ascribed to heredity and
the other to be ascribed to environment.

I have assumed that every individual is advanced or held back in his
development entirely independently of all the others. This hardly
corresponds to actual conditions. Comparing poor and well-to-do families,
all members of each family are obviously more or less exposed to
similar conditions. The poor are less favorably developed than the rich,
so that a secondary family resemblance develops not dependent upon
heredity but upon sameness of environment. The same may be said
of various parts of a population forming social strata or inhabiting different
geographical localities.

At present the real amount of the environmental influences to which
families are subjected cannot be determined.

We must furthermore remember that even those forms that reach
their final form at an early age are exposed to outer influences, and
that the values obtained by the assumption that the values observed
for early completed forms are maximal values express only a minimum
for the effects of environmental determinants. This is indicated by the
increased similarity of heterozygous twins which must be explained as
due to the sameness of their prenatal life.

It follows from these considerations that we may expect not only
differences between individuals due to exogene causes, but that there
will also be differences between populations due to the conditions under
which they live. We may also conclude from available data that among
individuals of the same descent such variations in stature may be measured
by a standard variability of ± 3-5 cm., so that in the same population
according to outer conditions differences in stature of several centimeters
may be expected. This is quite in accord with the observation
made both in Europe and America showing that stature has increased
by about 3 cm. during the last fifty years.

I repeat that the maximal value for similarity obtained from head
measurements must not be considered as an expression of hereditary
influences only, but that a certain amount of external influences is
contained in it. The same considerations that we made before may,
therefore, be applied, and, as we found that the stature in populations
must be variable according to environmental conditions although the
hereditary character of the population remains constant, so we may say
84in general that modifications of type may result from environmental
conditions without any fundamental, hereditary changes in the hereditary
character of populations. It seems to me that this explanation
of the changes observed in the United States is quite adequate and that
it is, for the present at least, unnecessary to look for hereditary changes.
It should be emphasized again that I have assumed here that the
degree of hereditary similarity for all characters is equal — an assumption
that may not be taken for granted without further investigation.
Thus it seems probable that in a mixed population, descendants of one
population with high and another with low head index, while the
hereditary element of stature is the same in either, the degree of similarity
for head index and for stature would be quite different. Nevertheless
the material used in this discussion seems to indicate that the
method can be used in fairly homogeneous populations. Extended
investigations on similarity are needed in order to show whether the
suggested approach is admissible.85

11 Zeitschrift für Ethnologie, vol. 45 (1913), pp. 622 et seq.

22 I have used here a value lower than the one found by Karl Pearson (Biometrika,
vol. 2 [1902-3], pp. 378, 379; see also E. Schuster, Ibid., vol. 4 [1905-6], p. 478),
since the following calculations are based on material which gives the value here
adopted (Franz Boas, Changes in Bodily Form, p. 156). The low value of the
regression is due to the character of the series which consists of Russian Jews, a
population mere homogeneous than Pearson's English material. See in regard to this
matter Franz Boas, The Mind of Primitive Man, 1938, pp. 60 et seq.

31 Here follows an attempt to evaluate the relative contributions of hereditary
and environmental determinants of variability which is here omitted.