6 STATISTICS OF SEX 



certain characteristic, A, are more likely than parents of the class not-A 

 to have children of either sex is determined by counting a sufficient num- 

 ber of the offspring of parents of each class and comparing the ratio of 

 male and female children in each class with the average ratio. If we 

 find this ratio to be markedly different in the two classes, we conclude 

 that the characteristic A is associated with some cause having a definite 

 effect upon the production of sex. 



Although the author proposes to apply this simple and obvious method 

 to certain cases in the following investigations, his main object is to go 

 farther, and inquire whether there are any causes or conditions what- 

 ever, known or unknown, which, in a decided degree, affect the produc- 

 tion of sex. When we see one family consisting mainly or wholly of 

 male children, and another consisting mainly or wholly of female chil- 

 dren, it is very natural to suspect that the excess, in each case, may be 

 due to some characteristic or faculty of the parents, or some peculiarity 

 of their constitution, which may or may not admit of discovery and 

 investigation. The mere fact of this inequality, taken in itself, does not, 

 however, prove anything, because it may be the natural result of those ac- 

 cidents which determine sex, but of which we know nothing. Granting 

 the existence of constitutional or other tendencies of the kind supposed, 

 we may apply the term unisexual to them. We may then make the hypo- 

 thesis that there is something in the constitution or habits of parents 

 which results in some having a unisexual tendency toward the production 

 of male children, and others toward the production of females, which 

 tendencies nevertheless elude investigation otherwise than b} r statistical 

 investigation of their effects. The desideratum is to discover a criterion 

 by which we may distinguish between inequalities in the division of a 

 family between the two sexes which are simply the result of chance and 

 those which are the result of a unisexual tendency on the part of the 

 parents. Such a criterion is pointed out in the first four sections of the 

 present paper, and its mathematical theory developed in the Appendix. 



2. THE PREPONDERANCE OF MALE BIRTHS. 



Certain facts preliminary to this inquiry may be set forth which will 

 serve as points of comparison in coordinating our conclusions. The 

 first of these is the well-known general fact that, in the entire Semitic 

 race, there is a small but well-marked preponderance of male over 

 female births. This preponderance is remarkably uniform in all Euro- 

 pean and American countries where complete statistics of births are 

 axailable. Mulhall finds the ratio to be 10.V2 male to 1000 female births 



