13 STATISTICS OF SEX 



SEX OF TWINS FRANCE AND BERLIN. 



Frauce Berlin 



1398-191 ID 1853-1880 



2 males 9,537 2,968 12,505 



Bisexual 9,826 3,489 13,315 



2 females 8,949 2,852 11,801 



Total males 28,900 9,425 38,325 



Total females 27,724 9,193 36,917 



M. to 100 F 104.2 102.5 103.8 



E m 2.1 1.2 1.9 



Normal bisexual 14,149 4,603 18,752 



Deficit 4,323 1,114 5,437 



The formal discussion, by algebraic methods, of the unisexual ten- 

 dency implied in these numbers will be found in the Appendix. These 

 methods are not, however, necessary to convey an idea of the principles 

 by which the results are to be explained. What makes a discussion of 

 these principles of interest is that the numbers derived from the statis- 

 tics of twins may be applied to the case of triplets, and a comparison of 

 the actual statistics of triplets with those derived from the statistics of 

 twins will be of interest. 



The processes which we presuppose are these: During an unknown 

 period of time, commencing with the moment of conception, the two 

 germs are exposed to a series of common influences, either in the male 

 or female direction, tending to make them of the same sex. As we can, 

 without appreciable error, make abstraction of the small normal prepon- 

 derance toward the male sex, we may say that, in the general average, this 

 unisexual tendency will be as often in one direction as in the other. Thus, 

 in one-half the cases, which we term group A, the common influences 

 preponderate in the male direction, and in other cases, which we call 

 group B, in the female direction. 



But these preponderating influences do not completely determine the 

 sexes. There are accidental causes operating differently on the two grow- 

 ing organisms, which may result in their becoming of opposite sexes. 

 Xumerically stated, the conclusion to which we are led is that the 

 statistics of twins may be explained by supposing that, in Group A 

 there is a probability of 0.77 in favor of either of the organisms taken at 

 random becoming male, and therefore 0.23 in favor of its becoming 

 female; while in group B there are similar probabilities in the oppo- 

 site direction. This, I say, is a conclusion from the statistics of twins, 

 assuming that there is no interaction between the two organisms tending 

 to make them of the same sex. I may remark, however, that this assump- 

 tion in the case of twins would have no special significance. The com- 



