RESEMBLANCE AND 



DIFFERENCE IN TWINS 



Twins That Look and Act Alike Attract Attention First, While Dissimilar 

 Ones Are Apt to Be Overlooked 



C. H. Danforth 

 Department of Anatomy, Washington University Scliool of Medicine 



AS EXPLAINED elsewhere in this 

 number of the journal, the 

 American Genetic Association 

 began some time ago to accumu- 

 late a collection of photographs of 

 twins. After a considerable number of 

 twins had been found, a questionnaire 

 was sent out to get data on resem- 

 blances and differences between the 

 members of twin pairs. It is the pur- 

 pose of this paper to discuss briefly 

 some phases of the question that were 

 emphasized by the answers received. 

 The writer is indebted to the president 

 of the Association for putting all the 

 material at his disposal. 



TWINS NOT ALWAYS ALIKE 



Some of the photographs reproduced 

 in this issue bear witness to the remark- 

 able degree of facial resemblance that 

 may exist between twins, while others 

 illustrate the fact that twins are some- 

 times surprisingly dissimilar. What the 

 photographs show with reference to 

 facial resemblance, the questionnaires, 

 filled out by the twins themselves, show 

 for other traits, both physical and men- 

 tal. While all types of twins are repre- 

 sented in the replies, there is some evi- 

 dence, as will be explained below, that 

 the material on hand comes from a 

 somewhat selected group. For this rea- 

 son it does not seem wise to present a 

 tabulated summary of the answers. 

 Such a tabulation would give the im- 

 pression that the average resemblance 

 of all twins is higher than it really is. 



The questionnaire was so planned 

 that it would aft'ord data for comparing 

 purely physical, more or less physio- 

 logical, and mental traits including 



likes, dislikes and normal reactions. The 

 answers may be briefly summarized 

 under general headings. 



Height. — Exact correspondence in 

 height is not unusual, and a difference 

 of a quarter of an inch or less is com- 

 mon. On the other hand, variations of 

 several inches sometimes occur, and 

 considerable differences are occasion- 

 ally found between twins that are other- 

 wise very similar. 



Weight. — The resemblances and dif- 

 ferences in weight are similar to those 

 for height, but there is no absolute cor- 

 relation between the two traits. Com- 

 plete identity, or a difference of only a 

 pound or two, is common. Some twins 

 state that, while their weight fluctuates, 

 the changes are always parallel so tha: 

 they never differ from each other by 

 more than a small amount. In othc; 

 cases there is a wide divergence even 

 when height and many other charac- 

 teristics are similar. 



Eye and Hair Color. — These features 

 are generally given as similar. For ex- 

 ample, one twin speaks of a "switch," 

 perfectly uniform in color, made from 

 the combings of both heads. Neverthe- 

 less twins, of which one is dark with 

 brown eyes, the other light with blue 

 eyes, are rather frequent. 



Other PJiysieal Traits. — So far as 

 can be judged by the answers, other 

 physical traits follow the same general 

 rule as those already mentioned — al- 

 most com]')lete identity in some cases, 

 and wide divergence in others. In gen- 

 eral, similarity with reference to one 

 trait is correlated with similarity in 

 reference to most other traits. Rarely 

 are there only a few points in common ; 



399 



