Changes in paternal testosterone across pregnancy, and hormonal linkage with the pregnant partner, may underlie fathers’ dedication to the partner relationship across the transition to parenthood, according to a study published online this July in Hormones and Behavior.

Many men show shifts in behavior from being mating-oriented to parenting-oriented during the transition to parenthood. From an evolutionary perspective this is advantageous but the biological mechanisms behind this are not well understood.

Testosterone, an androgenic steroid hormone, may play a role in men’s preparation for fatherhood. High levels of testosterone have been associated with competition and aggression, whereas low levels of testosterone have been associated with nurturance and particularly the care of offspring. Furthermore, lower levels of testosterone have been found in fathers who are in a relationship and fathers who are more involved in their children’s care, as compared to men without children.

Research has also shown that lower salivary testosterone is related to higher relationship satisfaction and commitment, lower interest in sex outside the marriage, and a lower likelihood of divorce. Therefore, it appears that a father’s decreases in testosterone levels around the transition to parenthood may reflect a shift away from pursuing new mating opportunities in favor of investment in offspring and the partner relationship.

The study, led by Darby Saxbe of the University of Southern California, followed 27 couples expecting their first child across their pregnancy and the first few months after birth. Parents provided repeated samples of testosterone across pregnancy to explore whether fathers’ change in testosterone, and the correlations with a mothers’ testosterone, were associated with levels of investment after the birth of their child. Thus, participants rated their investment, commitment, and satisfaction with their partner a few months after their child’s birth.

The results revealed that fathers showed significant declines in testosterone as the pregnancy progressed, as well as significant positive correlations with their partners’ testosterone level at each time-point. Moreover, the changes seen over the pregnancy and the degree of synchrony with mothers predicted the fathers’ investment, commitment, and satisfaction in the couple relationship. Interestingly, testosterone levels before the birth of the child predicted the relationship outcomes after the birth, even after adjusting for fathers’ scores of investment early on in the pregnancy.

The researchers concluded that, “The direction of our effects suggests that hormonal change and synchrony predict relationship investment, not the other way around; that is, relationship investment at the first prenatal assessment was not significantly associated with testosterone change or coordination with mothers.”