612 



CONTINUITY OF LIFE 



Fig. 24-19. A pair of identical or monozygotic twins. These two men are a result of a single zygote, hence their like- 

 ness. They are alike physically, mentally, ancJ even emotionally. Studies of such genetically identical individuals 

 has added much to our knowledge concerning the relative importance of heredity and environment. 



close similarity as might be expected, but 

 when such intangible traits as susceptibility 

 to disease (both organic and infectious), 

 food preferences, and even attitudes of 

 mind show marked correlations it seems 

 quite incredible. Intelligence tests usually 

 show that they vary no more than either 

 individual might on two different tests. 

 With such material as this, it should be pos- 

 sible to learn some interesting facts about 



o 



the effects of environment. 



Newman found in his studies that identi- 

 cal twins reared apart were over 90 per 

 cent alike in height, nearly 90 per cent alike 

 in weight, but only about 70 per cent alike 

 in intelligence as measured by standard 

 I.Q. (intelligence quotient) tests. Identical 



twins reared together had about the same 

 scores for height and weight but were also 

 nearly 90 per cent alike in regard to intel- 

 ligence. Two-egg twins (Fig. 24-18) of the 

 same sex were only about 65 per cent alike 

 in all of these traits. When one monozygotic 

 twin is bright, the other is also, but when 

 their educations vary considerably their 

 I.Q.s also vary correspondingly. Better 

 schooling definitely raises the I.Q. rating. 

 However, under similar cultural environ- 

 ments identical twins, although reared 

 apart, differed only 5 points in their I.Q. 

 ratings. This is within the normal variation 

 that might be expected from day to day 

 with the same person. From these studies, 

 environment seems to have a considerable 



