338 AN INTRODUCTION TO MODERN GENETICS 



genetical constitutions.^ Two fraternal twins have the same average 

 genetic difference as two ordinary sibs, but since their environments 

 are generally more similar than those of sibs, they tend to be more 

 highly correlated. Identical twins are in much the same environmental 

 situation as fraternals but in addition their genotypes are exactly the 

 same, and their correlation is correspondingly higher still. The simi- 

 larity between them extends to the most unexpected traits. Lange,^ for 

 instance, has shown that if one of a pair of identical twins has been to 

 prison, there is a very high likelihood that the other will commit some 

 crime and be caught. 



It is perhaps easiest to consider the resemblances between twins in 



Fig. 14-0. Criminality in Twins. — Notice that if one of a pair of identical twins 

 shows criminal tendencies, so does the other in a high proportion of cases. If the 

 twins are fraternals, it is more common to find only one of a pair affected. 



(After Holmes, from Rosanoff, Handy, and Rosanoff.) 



terms of average differences. The average difference in I.Q. between 

 fraternal twins in two well-known investigations was about 16-17 P^r 

 cent (Holtzinger,^ and Herrman and Hogben),* while that between 

 identicals was only 8-9 per cent. It seems that the variabihty between 

 twins is only cut in half when their genotj^^es are made exactiy the 

 same, so that roughly half the variability of the fraternals must be due 

 to the slight environmental differences between them. Such an estimate 

 of the magnitude of the effect should, however, be accepted with some 

 caution. If the tests are not quite reliable, we would find some variation 

 in score even if we measured the same individual on different days, 

 and a part of the variability of the identicals may be due to slight 

 imreliability of the tests. If this is so, the influence of the environment 

 wiU have been exaggerated in the estimate given above. But the facts 



^ For a review of the biology of twins, see Verschuer 1932. ^ Lange 1930. 

 ' Holtzinger 1929. * Herrman and Hogben 1933. 



