Students of child growth have been attempting for many years to determine and to express with quantitative exactness the relationship between the child's stage of physiological development and his status in several behavior manifestations.1 Since both types of measurement present difficulties, statements of precise relationship are rarely found. If physiological growth proceeded at an identical or even approximately equal rate for all children, the problem would be somewhat simplified. Enormous individual differences in rate of growth present important obstacles to exactness in quantitative measurement and expression of physiological maturity. Illustrative of the wide individual differences in physiological growth are data assembled by Engstr6m regarding age of first menstruation.2 Of 3,500 Finnish subjects studied by Engstr6m, 2 experienced their first menstruation at the age of 8 years, and I not until age 26. This range (18 years) shows clearly the individual differences in rate of physiological development; and this measurement expresses of course only one of the numerous elements involved in physiological maturity. In view of such wide differences in physiological growth and development, it seems plausible that differences in physiological maturation may be related positively to differences in the maturation of certain behavior manifestations. In a previous article, one of the writers presented data showing a positive relationship between the onset of pubescence in school girls and the decline of their interest in doll play.3 Using data assembled by the Baltimore Public Athletic League, the writer constructed a curve showing the age of first menstruation for certain American girls and a second curve showing the percentages of girls of various age levels who indicated that they had participated in doll play during the course of one week. The curves exhibited startling similarity. Furfey commented that it is particularly essential to know, not only the percentages of children of various ages who are interested in a certain activity, but also the percentage of participating prepubescents who continue to participate in the activity after the onset of pubescence.4 Furfey proceeded therefore to determine from his own data the number of I2-yr.-old boys who were interested