The influence of sex, of castration in new-born males and of androgen treatment in new-born females on the mouse-killing behaviour of the rat.

The results we have got on the mouse-killing behaviour of the rat point out the remarkable percentage difference between control males and control females, between integral males and males castrated at birth, and between normal females and females under the effects of androgens. Such results make us believe that the appearance of the killer behaviour is affected by the presence of testosterone in the first days of life of the rat.