Men with smaller testes make better fathers, researchers at the Emory University have found. The study also found a link between testicle size and nurturing-related brain activity in fathers.

Other research has shown that fatherly love is just as important to kids as mother's love and that peoples' willpower and determination can be traced back to their fathers' parenting style. The current study focused on finding out whether some men were biologically wired to take care of babies.

"Our data suggest that the biology of human males reflects a trade-off between mating and parenting effort," said anthropologist James Rilling from Emory University.

It is known than men with higher testosterone levels have high rates of divorce or polygamy while men with lower testosterone make better partners and caregivers.

In the present study, researchers measured the testicular volume of participants. The testes produce sperm along with testosterone.

The study included 70 men who were living with the biological mothers of their kids. Researchers interviewed all the couples separately to asses parenting styles of the men.

All the men in the study were then subjected to MRI scanning where they were shown images of their own kids with happy, sad or neutral expressions. They then measured the testicle volume via structural MRI.

Researchers found that men with smaller testis and lower testosterone were more involved in parenting than other men.

The MRI scans showed that activity in the ventral tegmental area (VTA) was directly co-related with testes volume. The VTA is a brain region that is associated with rewards and parenting.

Note that the study presents a correlation between volume of testes and parenting efforts. The researchers admit that though the correlation between the two was statistically significant, it wasn't perfect.

"The fact that we found this variance suggests personal choice," Rilling said in a news release. "Even though some men may be built differently, perhaps they are willing themselves to be more hands-on fathers. It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them."

Researchers also said that the arrival of children in the house changes the environment and could lead to shrinking testes in some men. A recent study had found that men who sleep near children experience a drop in testosterone levels. Lowering of male sex hormone is a signal that the body is adapting to parenting and devoting more time to understand the child's needs.

The study is published in the journal PNAS.