A couple of experiments by Dario Maestripieri, a University of Chicago biologist, offer insight into how early mothering affects our brain's chemical responses later in life -- as well as how we develop the urge to mother.
Since the 1990s, Maestripieri has worked with rhesus macaque monkeys. Socially, the rhesus aren't so much like humans. They're non-monogamous; females live in matrilineal groups, sharing food and casually lending a hand with each others' babies. Males hang out with each other, fighting for dominance; they occasionally stop by the females to copulate or steal food. But if you look at the attachment between mother and baby, Maestripieri thinks they're a perfect model: Rhesus females have one baby at a time, and they invest years in their care, just like humans do.
Observing the rhesus colony at the Yerkes Primate Center in Atlanta, Maestripieri noticed that the rhesus' mothering styles were as varied as humans'. Even before they became mothers, some females just loved to touch and hold babies, but some were about as maternal as Paris Hilton. When they had babies of their own, some of them doted, and some were downright abusive. So, he began a systematic look at how the rhesus' hormones changed with time and experience.
Maestripieri compared the mothers' estrogen, progesterone, and prolactin levels and tracked them over time, but found no differences. He tried manipulating their endogenous opioids, the beta-endorphins that get us naturally high. Nothing. Next, he looked at the role of early experiences. Right after birth, he switched around some newborn monkeys, giving the babies of good mothers to bad mothers, and letting the good mothers raise the babies of abusive females. The children of aggressive mothers tended to be aggressive themselves -- even though they were raised by sweet mothers. The same held true for sociability; the babies of irritable, unfriendly mothers tended to react to others the same way, even though they grew up in a cordial clan. This showed that an individual's tendency to be sweet or mean may be inherited.
But when Maestripieri looked at the brain chemicals of baby monkeys, he found that the kind of mothering they got did matter -- a lot. Some of the babies who were regularly rejected by their mothers -- being pushed away when they tried to climb into her arms, for example -- had up to 20 percent less serotonin, a neurotransmitter that's a mood elevator. Low levels of serotonin are associated with depression, anxiety, and impulsive aggression in monkeys and humans. The more rejection a baby experienced, the less serotonin it produced, and these low levels continued into adulthood. Some of these low-serotonin monkeys went on to become bad mothers themselves.
While serotonin isn't a direct part the oxytocin/attachment system, the two brain chemicals are closely related. Serotonin stimulates the release of both oxytocin and vasopressin. Therefore, it's a good bet that monkeys and humans with low levels of serotonin don't experience as strong an oxytocin response. They may not bond as deeply; they may not be able to bond at all.
About half of the abused monkey babies, however, went on to become relatively good mothers. And they didn't have lowered serotonin levels. It's possible that they inherited more resilience to stress and more oxytocin-rich parasympathetic nervous systems from their loving mommas. This is reassuring to all of us who didn't get the kind of mothering we wish we had. We can overcome both nature and nurture to raise children who are even more secure and more loving than we are.
If, like me, you're fascinated about how nature and nurture made us the way we are -- and why we do or don't turn out like our parents, it's well worth searching out his research. Here are the studies I've covered:
Maestripieri, Dario. 2003. Similarities in Affiliation and Aggression Between Cross-Fostered Rhesus Macaque Females and Their Biological Mothers. Developmental Psychobiology 43(4):321-327.
Maestripieri, Dario. 2005. Early experience affects the intergenerational transmission of infant abuse in rhesus monkeys. Proceedings of the National Academy of Sciences 102:9726–9729.