THE HISTORY OF. TWINS, ETC. 345 



THE HISTOEY OF TWIjS^S, AS A CKITEKION OF THE 

 RELATIVE POWERS OF KATURE K^T> NURTURE.' 



By FRANCIS GALTON, F. R. S 



rriHE exceedingly close resemblance attributed to twins Las been 

 * JL the subject of many novels and inlays, and most persons have 

 felt a desire to know upon what basis of truth those works of fiction 

 may rest. But twins have many other claims to attention, one of 

 which will be discussed in the present memoir. It is, that their 

 history aifords means of distinguishing between the efiects of ten- 

 dencies received at birth and of those that were imposed by the 

 circumstances of their after-lives ; in other words, between the effects 

 of nature and of nurture. This is a subject of especial importance 

 in its bearings on investigations into mental heredity, and I, for my 

 part, have keenly felt the difiiculty of drawing the necessary dis- 

 tinction whenever I tried to estimate the degree in which mental 

 ability was, on the average, inherited. The objection to statistical 

 evidence in proof of its inheritance has always been : " The persons 

 whom you compare may have lived under similar social conditions 

 and have had similar advantages of education, but such prominent 

 conditions are only a small part of those that determine the future 

 of each man's life. It is to trifling accidental circumstances that the 

 bent of his disposition and his success are mainly due, and these you 

 leave wholly out of account in fact, they do not admit of being 

 tabulated, and therefore your statistics, however plausible at first 

 sight, are really of very little use." No method of inquiry which I 

 have been able to carry out and I have tried many methods is 

 wholly free from this objection. I have therefore attacked the prob- 

 lem from the opposite side, seeking for some new method by which 

 it would be possible to weigh in just scales the respective effects of 

 nature and nui'ture, and to ascertain their several shares in framing 

 the disposition and intellectual ability of men. The life-history of 

 twins supplies what I wanted. We might begin by inquiring about ' 

 twins who were closely alike in boyhood and youth, and who were 

 educated together for many years, and learn whether they subse- 

 quently grew unlike, and, if so, what the main causes were which, in 

 the opinion of the family, produced the dissimilarity. In this way 

 Ave may obtain much direct evidence of the kind we want ; but we 

 can also obtain yet more valuable evidence by a converse method. 

 We can inquire into the history of twins who were exceedingly unlike 

 in childhood, and learn how far they became assimilated under the 



' In my "English Men of Science," 1874, p. 12, I treated this subject in a cursory 

 way. It subsequently occurred to me that it deserved a more elaborate inquiry, which 

 I made, and of which this paper is a result. 



