HUMAN GENETICS 337 



this compares with the range of mentality which is found. For any- 

 thing Hke an exact analysis, some measure of intelligence is necessary. 

 It is usual to use one or other of the "intelligence tests" devised by 

 psychologists starting with Binet.^ No one knows exactly what these 

 tests actually measure, but the measures are fairly reproducible when 

 the same individual, or better, the same group of individuals, is measured 

 at different times, so that the thing which is measured seems to be 

 something fairly definite, and is presumably one of the factors referred 

 to when the word intelUgence is used in the ordinary unanalysed way. 

 To allow for age, the measure is usually expressed as the IntelUgence 

 Quotient or I.Q., which is the actual measured inteUigence expressed 

 as a percentage of the average intelligence at that age. 



Taking the I.Q. as the variable, we can discover the correlations 

 between different pairs of relations. Between pairs of sibs (brothers 

 and sisters) of the same sex, it is usually about o • 4^ and gives no 

 indication of the influence of sex-linked genes, which would tend to 

 cause a higher correlation between female pairs than between males. 

 But that is all that can be deduced from the data on ordinary sibs. To 

 progress any farther we must make uniform either the environments or 

 the genotypes. A moderately uniform environment is provided when 

 children are, at a very early age, adopted into an orphanage or other 

 institution, though even then the uterine environments of successive 

 children in a family are probably by no means the same. The facts 

 about orphanage children, however, are not very certain. Some investi- 

 gators have found a high correlation between sibs reared in an insti- 

 tution (e.g. "53 for Gordon's data analysed by Elderton),^ others have 

 found a correlation no higher than that between normal children (e.g. 

 Davis). ^ The results of the reverse process, increasing the environ- 

 mental differences between sibs by having them adopted into different 

 homes, seem to indicate an effect of the environment, since the correla- 

 tions are usually lowered.^ But this method hardly leads to quantitative 

 estimates and the data are still scanty. 



We can obtain rather fuller information from cases in which the 

 genotypes are the same, i.e. from identical twins. As is well known, 

 there are two sorts of twins, fraternals, which are ordinary sibs which 

 happen to be conceived at the same time, and identicals, both of which 

 proceed from the same fertilized egg and therefore have identical 



^ Cf. Dearborn 1928. 2 g g Herrman and Hogben 1933. 



' Elderton 1922. * Davis 1928. 



^ Freeman, Holtzinger, and Mitchell 1928. 



