UNISEXUAL TENDENCY 9 



4. INQUIRY WHETHER ANY UNISEXUAL TENDENCY, PERMANENT 

 IN THE INDIVIDUAL IN EITHER DIRECTION, EXISTS AMONG 

 PARENTS. 



The well-known fact being that inequalities in the proportion of the 

 two sexes are almost the universal rule, some families consisting mainly 

 or entirely of male children, others mainly or entirely of female chil- 

 dren, the question before us is whether these inequalities are simply the 

 result of chance, or show unisexual tendencies on the part of the respective 

 parents. What we want is a criterion for distinguishing between these 

 two cases. To make clear the principle of the proposed criterion, we begin 

 with an illustration of its application. Let us suppose that we select at 

 random 100 families of two children each. Granting that this selection 

 corresponds to the general average, and leaving out of consideration the 

 small preponderance of male births, we shall have, in these families, 50 

 cases in which the first-born was a male, and 50 in which it was a 

 female. Of these two sets of 50 each, if there is no unisexual tendency, 

 the second child will be a male in one-half, or 25 cases, and a female in 

 the other cases. If there is no tendency of the kind sought, the final 

 result will be : 



25 families of 2 females each; 

 25 families of 2 males each; 

 50 of 1 male and 1 female. 



That is to say, in the great mass the number of families comprising 

 two children of the same sex will be the same as that of families com- 

 prising two children of opposite sexes. 



Now let us suppose that, in consequence of some cause not known in 

 advance, some parents have a tendency toward the production of male 

 and others toward the production of female children. To fix the ideas, 

 let us suppose that in the case of one-half of the parents, which we call 

 Class A, there is a probability of three-fifths in favor of a male child, 

 and in the remaining half, called Class B, a similar preponderance in 

 favor of a female child. The method presupposes that we have no clue 

 to a decision as to which of these classes any given parents belong. The 

 first-born will still be a male in one-half and a female in the other half 

 of the cases. But, in case of the second child, the 50 parents of Class A 

 will have 30 male and only 20 female children. The 50 of Class B will 

 have 20 male and 30 female children. The probable distribution of 

 children between the two classes will be as follows : Of the 50 parents 

 of Class A, 30 will have male first-born children, and 20 will have female 

 first-born. The preponderance being the same in the cases of the second 



