"The general idea is that lower testosterone on a day-to-day basis helps attune fathers to the needs of their children," University of Notre Dame anthropologist Lee Gettler, who studies this effect, told NBC News.
Lower testosterone may also make men more empathetic, less aggressive, less interested in mating, or all these.
The idea is part of Life History Theory. The theory holds that many animals, including people, trade off between putting resources into mating, versus parenting. The more energy devoted to having sex, and engaging in competition with others to do so, the less that's left for raising offspring, and vice versa.
The life histories of children have shown that the more stress and family disruption they experience, the greater the risk they'll face troubles later. Girls with an absent father, for example, are more likely to start their periods sooner, have sex sooner, and to become single mothers. Boys with absent fathers or stressful childhoods are more likely to begin having sex earlier. They are more likely to put more effort into mating, less into parenting.
The Emory group, led by post-doctoral fellow Jennifer Mascaro in the lab of James Rilling, is the first to use testicle size as a physical marker, and to see if testicle size correlates with brain reward – positive feelings -- from nurturing as a way to help explain variation in male parenting.
Image: Happy father and son, via Shutterstock