In a first study, we investigated how the absence of a father and the presence of a stepfather during early childhood affected physiological and behavioral traits related to reproductive development (such as age of menarche, age of first sexual intercourse and number of sexual partners) in a large sample set of male and female French university students. We evaluated which ages were sensitive to modifications in the family composition and found that menarche occurred earlier when the father was absent, particularly when the child was between 0 and 5 years of age. Father absence during early adolescence was associated with a younger age at first sexual intercourse and an increased number of sexual partners, for both sexes. The presence of a stepfather during this period further advanced the age of first sexual intercourse. We also measured testosterone levels in both sexes and analyzed their association with parental separation, and found that young women with separated parents had significantly higher afternoon levels of testosterone. In a second study, we analyzed direct fitness measures (such as number of children and grandchildren) in a large sample of French workers and found that parental separation during childhood was not associated with fitness variation. We discuss whether the reproductive outcomes of individuals having experienced modifications in the early family environment are the expression of costs or adaptive strategies.