(CNN) A new study from the National Academy of Sciences suggests that dad's with smaller testicles are more likely to be nurturing to their babies. Researchers used an MRI to study the testes of 70 biological fathers and their brain patterns as they looked at pictures of their children, other children, and adults. What they found was the fathers' testicular size and testosterone levels were "inversely related to parental investment." They say this does not mean that men with small testes are better dads. They say there are a lot of other things that go into being a father.. like a sense of humor and lots of patience.