Our upbringing greatly influences us, often determining our life’s trajectory as adults. But our parents don’t just shape our values and aspirations — they can even alter our biology, which in turn can modulate behavior. Case in point, a new study found that boys whose fathers were present and involved with their upbringing when they were teens had lower testosterone levels when they later became fathers in their own turn, compared to boys who lived in households with an absent father.

The new study was led by Lee Gettler, an anthropologist at the University of Notre Dame who previously made headlines when he showed that the transition to fatherhood causes new dads’ testosterone levels to sharply drop. Those findings were very insightful, showing how men are, at least to a certain degree, biologically hardwired to care for their children. Now, this new study shows that adolescence is a critical period during which social relationships between a boy and his father can influence later hormone production, with far-reaching consequences later in life.

Deadbeat dads in nature

The vast majority of male mammals have a shoot-and-scoot mating strategy, mating with as many females as they can without bothering to participate in the rearing of the offspring. Males provide paternal care in only 5% of mammalian species. That’s despite the fact that parenting is obviously evolutionary beneficial to both partners as it enhances their common offspring’s health and survival prospects.

You only need one caring parent, though. As a result, differences in the costs and benefits of caring for offspring have been, over time, exaggerated by natural selection. For instance, in marsupials, females are the sex that has evolved pouches, greatly enhancing their ability to care for their young – so the females do most or all of the childcare.

It may look like females got the short end of the stick, but things are rarely out of balance in nature. Since females need to devote more resources to raising their young, they’ve also become very picky with their mate selection. As a result, males will typically face enormous sexual selection pressure. Oftentimes this means that a large fraction of males in a population never sires any young. Paradox solved.

However, in humans, both sexes tend to be involved in caring for their young, though their roles are different. For instance, women have mammary glands that would, theoretically, allow them to care for their children all by themselves if the situation called for it. But usually, that’s not really enough. Human beings are pretty helpless until they reach puberty, and having a father fulfilling at least a provider role can be critical to the survival of children. That’s usually enough incentive for the father to stick around, although we’ve all seen cases and cases.

The notion that humans are outliers and that fatherhood may be hard-wired in our biology has only recently come to light. In the 1990s, a study on US Army veterans showed that married men had significantly lowered testosterone levels than their single counterparts, and some even used a testosterone booster supplement. Another study on Air Force veterans indicated that men’s testosterone levels increased around the time of divorce.

But it wasn’t until recently that the link between testosterone production and fatherhood was put into an evolutionary perspective. In 2011, Gettler and colleagues studied a community-based sample from the Philippines, showing that men with higher testosterone were more likely to get married, but also that fatherhood caused their testosterone to decrease.

This is not all that surprising. It’s an established fact that hormones have a huge impact on our mood and behavior. Think of the behavioral switches of puberty or even your castrated pet. And this relationship seems to be a two-way street, in that behavior can also influence hormones.

Dads, teens, and testosterone: what’s the common thread?

In a new study, Gettler and colleagues analyzed data from almost 1,000 men in the Philippines spanning more than 30 years. The men were basically enrolled in the study as babies, and their health and nutrition were regularly monitored over the years. The collected data also included information on whether the participants’ fathers stayed around, as well as the degree of paternal care that they offered. This data provided a unique opportunity for the researchers to look for patterns in a chaotic environment, bringing them as close to a natural experiment as possible.

“There are very few studies that have looked at how early-life social experiences with family — and dads especially — are related to future testosterone production in men, including when they become fathers,” Gettler said in a statement. “There are none that have considered the potential role of the adolescent time period and take into consideration what boys have experienced with their dads. The longitudinal part of this large study is really key because it has tracked participants since they were infants. In adolescence, the boys began to contribute their own perspectives, whereas their mothers were answering surveys in their younger years. This way we know who each teenage boy credited for his upbringing. We also get their perspective when they later become fathers regarding how involved they are with caring for their own children.”

The researchers found that teenage boys whose dads were more involved in their upbringing had lower testosterone as future fathers. The study also found that boys whose fathers were present but not involved with childcare grew up to have higher testosterone, on average, than boys who grew up in households where the dad was more involved. Testosterone plays a major role in men’s health, offering protection against cardiovascular disease and improving immune function.

“So, these findings are showing us new ways that family experiences before adulthood can shape later biology that, in turn, can affect later behavior and health,” Gettler said.

But what does being an “involved” dad actually mean? There’s no real right answer, and some aspects of fatherhood need to be contextualized. When this study began in 1983, most of the fathers worked as farmers, fishermen, or tradesmen, who would often work long hours. But although they didn’t spend much time at home compared to their mothers, dads in the Philippines usually act as moral guides and disciplinarians.

“Some activities, like playing sports or teaching skills — the things that involve more direct interaction —would fall under the designation ‘dad was involved with care’ in our study. Other domains, like being a role model or a moral guide, might be categorized as ‘the presence of a dad’; they are still present and contributing to the sons’ lives. There’s not just one way to be a good dad,” Gettler said.

If you were to ask a doctor which factors may diminish a man’s testosterone, chances are that things like age, sleep, obesity, or diabetes would come up. A man’s relationship status is unlikely to arise in the discussion, but as we’re beginning to learn, a man’s “partnership status” and “fatherhood” could be really important and have far-reaching consequences in terms of behavior for both fathers and boys.

At the same time, the findings also raise some intriguing questions and speculations. Low testosterone may help men bond better with their children and be less inclined to “jump ship”, so to speak. When these adolescents grow up and have families of their own, their biology — which influences their behavior — may make them inclined to act the same. But high-testosterone fathers who don’t stick around will sire high-testosterone babies that will be inclined to do the same. In both situations, you have self-enforcing cycles, with no clear delineation of where genetics and nurture begin and end.

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