According to research published in the Journal of Personality and Social Psychology, being involved in a romantic relationship reduces levels of testosterone in men, but only if they are committed to monogamy in that relationship.
Previous studies have examined the changes caused by beginning or ending a romantic relationship. They found that being in a relationship tended to lower testosterone levels in men while ending a relationship tended to cause higher testosterone levels.
This study, on the other hand, examined the association between being interested in finding new sexual partners, whether already in a romantic relationship or not, and testosterone levels.
The authors of this research conducted two separate experiments to better understand this association.
In the first experiment, the authors examined the saliva samples of 102 male college students. These students also completed the Sociosexual Orientation Inventory, a questionnaire that assessed their sexual behavior, current relationship status, and willingness to seek out new partners.
In the second experiment, 69 male students provided a saliva sample and completed the Sociosexual Orientation Inventory, but were also asked if they would consider having an affair in the future and if they had previously cheated on their partner.
Both studies found that men committed to sexual exclusivity with their partners had lower testosterone levels.
In contrast, men who were involved in a romantic relationship but were still interested in finding new sexual partners did not show a reduced level.
This suggests that changes in testosterone levels are not due to being in a relationship or not, but rather are influenced by interest new sexual partners.
The authors of this research believe this decrease in testosterone when men are not interested in seeking out new sexual partners may be related to parenting.
“In conjunction with evidence that lower testosterone facilitates parental care, these findings suggest that human men, like some birds, do indeed have special biological adaptations that promote parental investment by diverting reproductive efforts away from mate seeking.”
McIntyre, M., Gangestad, S.W., Gray, P.B., Chapman, J.F., Burnham, T.C., O’Rourke, M.T., Thornhill, R. (2006). Romantic involvement often reduces men’s testosterone levels – but not always: The moderating role of extrapair sexual interest. Journal of Personality and Social Psychology, Vol 91, No 4: 642-651.