Prolactin is a hormone whose two most well-known effects in humans are its regulation of milk production for suckling mothers, and its lowering of the levels of sex hormones, testosterone and estrogen, for male and female, respectively.
In a recent publication , L. T. Gettler et al write about the relationship of fatherhood with Prolactin levels in males, specifically those who are living in Cebu. Prolactin (PRL) being related to the nurturing behavior of mothers, is a good candidate hormone that according to the authors, “may increase with fatherhood to actively facilitate behavioral or emotional states related to successful childcare.”
The male in the study came from the Cebu Longitudinal Health and Nutrition Survey, a study of a cohort of Filipino women who gave birth between May 1, 1983, and April 30, 1984 . In 2005, the male sample were between 20.8 to 22.6 years at the time of data collection. Another set of data sampling was done on the same sample in 2009.
They found out that the Prolactin levels of nonfathers are significantly lower compared to fathers.
Moreover, the authors note that fathers of infants (1 year old or less) had higher prolactin. However when they stratified the men according to being first-time or more experienced fathers, there was not a difference. L. T. Gettler, the lead author, said, “this suggests that something about being a father to an infant (rather than being a first-time father, for example), might be raising men’s prolactin in this sample of Cebuano men.”
The authors also said that they could not distinguish PRL levels between fathers who are more involved with childcare and fathers who are less involved.
Did fatherhood increase PRL levels? or Are men with higher PRL levels just more likely to sire?
According to the authors, it would seem that based on their results, fatherhood causes the increase. But they also said that further studies are needed to be done.
The nonfathers´ was a curious case. The authors hypothesized that nonfathers with higher PRL levels are more likely to have less sexual partners and sexual activity. However, they noticed the contrary. These nonfathers have more sexual partners and more sexual activity compared to nonfathers with lower PRL.
The authors could only guess why. They said that based on other literature: 1) PRL stimulates the immune system which may increase men’s attractiveness; and 2) PRL alters men’s personality that increases attractiveness to potential mates.
The authors conclude that based on their results, “the role of Prolactin in human male reproductive strategies may be more complex than previously conceived and hint that the function of the hormone may shift as men move through different life history stages.”
 Gettler LT, McDade TW, Feranil AB, & Kuzawa CW (2012). Prolactin, fatherhood, and reproductive behavior in human males. American Journal of Physical Anthropology, 148 (3), 362-70 PMID: 22576125