A US study Monday measuring fathering habits and testicle size suggested that bigger may not be better when it comes to the day-to-day raising of small children.

The research involved 70 U.S. 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 diapers, 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 21-55 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 diapers, 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.

"If you invest more energy in parenting you have less available for mating and vice versa," explained Rilling.

Since the testes are where sperm is made, and their size can be linked to the amount produced, the researchers said their study is unique and the first of its kind.

Previous studies have shown a link between high testosterone levels and lower parental involvement as well as divorce and infidelity. The Emory team also analyzed testosterone levels and found the same inverse relationship to parental involvement in their study.

"Other people have looked at testosterone and parental behavior but as far as we know we are the first to look at testes size and parental behavior and we think we are getting at something different," said Rilling.

"We are suggesting that men with larger testes are more built for a mating effort strategy and as a consequence are less built for investing in children."

Researchers used functional MRI scans to analyze brain activity when the men were shown pictures of their toddlers and also of strangers' children.

To assess the men's daily parenting involvement with their young children, scientists asked the men and their female partners to separately fill out questionnaires.

The volume of the testes was measured in a voluntary MRI scan, to which 55 of the 70 men agreed.

Still, the researchers could not say for sure whether testes size caused the difference in fathering behavior, or if perhaps the act of becoming a father might have caused the testes to shrink in some men.

Urologist Joseph Aluka, who was not involved in the research, said he commonly sees men with smaller testes in a certain context.

"The guy who comes in with smaller testes is more likely to have greater difficulty with getting his wife pregnant," Aluka, an assistant professor at New York University Urology Associates, told AFP.

If such men end up being more involved as parents, "maybe these guys struggled to have kids and appreciate the experience a little bit more," Aluka said.

"I wouldn't be surprised if just a few participants in this study fundamentally affected their data because it is a small study," said Aluka, describing the findings as "a stretch."

The study appeared in the Proceedings of the National Academy of Sciences.

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  • “I believe that what we become depends on what our fathers teach us at odd moments, when they aren't trying to teach us. We are formed by little scraps of wisdom.” - Umberto Eco

  • “Fatherhood is great because you can ruin someone from scratch.” – Jon Stewart

  • "Above all, children need our unconditional love, whether they succeed or make mistakes; when life is easy and when life is tough." -President Obama

  • “When I was a boy of 14, my father was so ignorant I could hardly stand to have the old man around. But when I got to be 21, I was astonished at how much he had learned in seven years.” – Mark Twain

  • “The nature of impending fatherhood is that you are doing something that you’re unqualified to do, and then you become qualified while doing it.” – John Green

  • "A new father quickly learns that his child invariably comes to the bathroom at precisely the times when he's in there, as if he needed company." - Bill Cosby

  • "You fathers will understand. You have a little girl. She looks up to you. You're her oracle. You're her hero." -Stanley T. Banks, Father of the Bride

  • "Do I want to be a hero to my son? No. I would like to be a very real human being. That's hard enough." -Robert Downey Jr.

  • “It doesn’t matter who my father was; it matters who I remember he was.” — Anne Sexton

  • "I thought I would be more inspired to have all these new feelings to talk about, but I really just want to hang out with my daughter." Jay-Z

  • “We never know the love of a parent till we become parents ourselves.” - Henry Ward Beecher

  • “By the time a man realizes that maybe his father was right, he usually has a son who thinks he’s wrong.” - Charles Wadsworth

  • "Having a staring contest with a newborn is one of the weirdest things you will ever do. And it is highly recommended." -Ross McCammon

  • “I think once I fail enough as a dad, I'll be looking for help wherever I can get it. I just need enough time to screw things up and then I'll start looking to TV dads for advice.” - Ty Burrell

  • "The reward of child rearing is spending the rest of your life proudly knowing this person you helped guide. Let him be himself." -Mike Sager

  • "I want my son to wear a helmet 24 hours a day." - Will Arnett

  • "This is my most important role. If I fail at this, I fail at everything." -Mark Wahlberg

  • "It is admirable for a man to take his son fishing, but there is a special place in heaven for the father who takes his daughter shopping."- John Sinor

  • “I have found the best way to give advice to your children is to find out what they want and then advise them to do it.” – Harry S. Truman

  • "It is much easier to become a father than to be one." -Kent Nerburn, Letters to My Son: Reflections on Becoming a Man

  • "Lately all my friends are worried that they're turning into their fathers. I'm worried that I'm not." -Dan Zevin

  • "Few sons are like their fathers - many are worse, few better." -Homer, The Odyssey

  • "It was times like these when I thought my father, who hated guns and had never been to any wars, was the bravest man who ever lived. "-Harper Lee, To Kill a Mockingbird

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