A STATISTICAL INQUIRY INTO THE PROBABILITY OF CAUSES 

 OF THE PRODUCTION OF SEX IN HUMAN OFFSPRING. 



BY SIMON NEWCOMB. 



1. Introductory 5 



2. Preponderance of male births 6 



3. Is the ratio of male to female births the same in all races? 7 



4. Inquiry whether any unisexual tendency exists among parents 9 



5. Unisexual tendency in multiple births 16 



6. Processes suggested by the statistics of multiple births 21 



7. Influence of the age of the parent on sex 22 



8. Supposed influence of other conditions 26 



9. Summary of conclusions 28 



APPENDIX. Mathematical theory of the effect of unisexual tendency. ... 29 



1. INTRODUCTORY. 



The object of the present investigation is to make an application of 

 certain hitherto undeveloped statistical methods to a class of cases in 

 which the causes elude all direct inquiry. The subject of such methods, 

 although its general principles are well understood, is one which is sus- 

 ceptible of being worked out in detail to a far greater extent than has 

 yet been seriously attempted. The present paper may therefore be 

 regarded as having a double object: one to illustrate and apply certain 

 methods; the other to draw conclusions on a physiological question of 

 the widest scientific and human interest. 



The idea that the sex of offspring may depend, to a greater or less 

 extent, on discoverable causes, perhaps even on causes within the control 

 of the parents, is a very natural one. Examples of what might be pos- 

 sible causes are the respective ages of the parents, their vigor or condition 

 of health, their conjugal habits, which may be infinitely varied, or the 

 relation of the time of conception to the periods of menstruation. 

 Besides this, it is possible that some parents, male or female, may be 

 endued with some special faculty for producing children of one sex 

 rather than another, owing to constitutional or other causes which, not 

 being apparent on the surface, might entirely elude direct investigation. 



The method of investigating this tendency in the case of recognizable 

 possible causes is well known. The question whether parents having a 



