A US study measuring fathering habits and testicle size suggests that bigger may not be better when it comes to the day-to-day raising of small children.
The research involved 70 US men of varying ethnicities - most were Caucasian, five were Asian and 15 were African-American. All were the fathers of children aged one to two.
The larger the volume of their testes, the less the men were involved in daily parenting activities like changing nappies, said the study by researchers at Emory University in Georgia.
In comparison, men with smaller testes showed more nurturing activity in the brain when shown pictures of their children, and also were more involved in their children's upbringing, according to surveys answered separately by both the fathers and their female partners.
All the men in the study were aged 21 to 55 and lived with the biological mothers of their children. Most were married.
"I wouldn't want to say that men with large testes are always bad fathers but our data show a tendency for them to be less involved in things like changing nappies, bathing children, preparing meals, taking them to the doctor and things like that," said lead author James Rilling, an associate professor of anthropology.
The study sought to test an evolutionary theory that holds that people and animals are either built to breed or to nurture.
The findings support the notion that human beings have a limited amount of energy to invest in reproductive efforts - so either they put energy into producing offspring or into raising it.
Previous studies have shown a link between high testosterone levels and lower parental involvement as well as divorce and infidelity. The Emory team also analysed testosterone levels and found the same inverse relationship to parental involvement in their study.
Still, the researchers could not say for sure whether testes size caused the difference in fathering behaviour, or if perhaps the act of becoming a father might have caused the testes to shrink in some men.
The study appeared in the Proceedings of the National Academy of Sciences.