At playgrounds, I like to mom-watch and wonder: What makes each of us tick? Why is that mom hissing at her toddler like a snake? Why is this one hunched with a highlighter and a stack of papers, as if she were in a corner office instead of in the crook of the twisty slide? And that one over there — how can she possibly have the patience to pretend-eat yet another ice cream cone made of sand?

Then of course there’s me, spying on these other women — and some dads, too — instead of keeping tabs on my four kids, one of whom is poised to brain somebody with a stick.

Although the mental transition into motherhood is universal among mammals, each human mother is also unique in measurable ways.

We are worlds away from understanding the precise nature of these differences, let alone what causes them, and much more research is needed. But scientists hypothesize that mothers are shaped by a slew of influences, some abstract (culture, stress) and others specific (mode of delivery). A few of these ideas are controversial, none are definitive and some are downright bizarre: One study found that having played a musical instrument boosted parents’ sensitivity to infant cues. Yet contemplating the behind-the-scenes powers shaping the maternal mind may help moms have more sympathy and understanding for one another — not to mention for ourselves.

Your childhood caregivers

Scientists over decades have observed how parental behavior is repeated in families, with kids growing up to mirror their parents. The mother-daughter relationship may be especially influential, with a neuroimaging study from Yale University showing extra gray matter in the brains of moms who reported having warm relationships with their own mothers. These same women were also more sensitive to infant cues.

Yet such patterns may also defy bloodlines. Studies in rats and monkeys suggest that cross-fostered young grow up to resemble their adoptive mothers, rather than their biological moms. The nurturing you receive is at least as important as your nature.

Your babysitting history

There is some evidence that prior experience with young children gives women — and men — more confidence in their parenting. In one laboratory test, a background in child care made it easier for women to diagnose the cause of a baby’s crying. In another experiment involving our primate kin, young chimpanzees that had been exposed to baby chimps while pregnant had an easier time when they became mothers compared with peers that had no such interactions. These previous exposures may help prime maternal circuitry, some intriguing work in lab rodents suggests.

In certain species, previous exposure to other babies is a prerequisite for a mother to successfully care for her own. If real babies are scarce, toy babies may do: Keepers at the Smithsonian’s National Zoo hand out little plush gorilla babies to stimulate pregnant first-time mothers, as part of their “maternal training.” The gorilla dolls have electronics inside for realistic baby babble.

Your mode of delivery

Moms who deliver by Caesarean section (four and counting, in my case) may face hidden challenges. Some research shows that they’re slightly less sensitive to infant cries after delivery. Women who’ve had emergency C-sections (I’ve been there, too) are 15 percent more likely to suffer from postpartum depression, one study found.

These moms’ challenges may have to do with their higher degree of postpartum pain. But they may also be missing some of the chemical benefits of vaginal delivery: In many mammals, vago-cervical stimulation during labor unleashes an important cascade of neurochemicals that influence the maternal mind. Happily, for human mothers, the hormonal differences between vaginal and caesarean delivery seem to diminish as you spend time with your baby, which makes the neurochemicals cascade as well.

Your social support

Women who feel supported by loved ones on average have easier births, fewer C-sections, more robust newborns, lower rates of postpartum depression and better luck breastfeeding, according to an article in the journal Human Nature. Maternal grandmothers are an important source of this support, but a loving romantic partner is also crucial. A study of a newly instituted paternal leave policy in Sweden showed that the extra 30 days dads spent at home tracked with a 26 percent decrease in the amount of anti-anxiety medications that new moms obtained, possibly because they were emotionally buoyed by the dad’s presence.

Absent partners, on the other hand, are linked with premature birth, maternal anemia, high blood pressure and depression. Some mental health difficulties of single moms may be explained by compromised access to social networks and other resources. But friends and family members can step in to widen those networks.

And, of course, a toxic or abusive relationship with a partner can be more harmful to mothers and their children than no partner at all.

Your bank account

During periods of economic anxiety or recession, pregnant women are more likely to miscarry or go into labor prematurely. Amid the subprime loan crisis, foreclosed-upon moms bore lower-weight babies. And public health researcher Timothy Bruckner of the University of California at Irvine calculated that a 1 percent drop in employment in a given California city predicts an 8 percent increase in “infant mortality due to unintentional injury” that same month. In addition to sudden swings in financial circumstances, chronic socioeconomic disadvantage and its stresses can leave a mark as well, changing women’s neural responses to infant cues.

Your history of trauma

Trauma may subtly change how mothers’ brains function. A study of Israeli moms who’d lived for years near the Gaza border under constant threat of rocket fire showed their brains were less active in regions related to empathy. Pregnant women and mothers of young children are also at high risk for mental health problems in the stressful wake of environmental disasters, like earthquakes and nuclear power plant meltdowns.

The traumatic event doesn’t have to make headlines; it could be difficult childhood memories or an abusive past relationship. In fMRI tests, the brains of American mothers who reported a history of such private traumas look slightly atypical when gazing at the sad faces of their own children. Women who had no trauma history exhibited a strong response in the brain’s amygdala — but traumatized women show a blunted response, which researchers characterized as “a possible disengagement from infant distress” and a potential sign of “disrupted maternal caregiving.”

Scientists are now studying whether therapy can reshape some of these processes, with preliminary evidence suggesting that it may.

Your number of kids

First-time and veteran moms aren’t the same, especially in the first few months. That’s probably because the first-timers are undergoing the brain change of motherhood, which scientists suspect might be cumulative, deepening with subsequent kids. Experienced moms may more “efficiently and effectively differentiate and respond to infant emotional cues” like crying, one study found. They tend to touch infants more, have an easier time bonding and a lower risk of postpartum depression.

Then again, some reports suggest first-time moms may not suffer as much from momnesia, the foggy mom brain that many women report. These women may also be more attentive and motivated to make up for their learning curve, which could explain why firstborns outperform later siblings in school and are overrepresented at elite colleges.

Your culture

So many key maternal variables — a spouse’s role, the distance to the nearest grandparent — are socially inflected. In some parts of the world, moms do not regularly play with their kids or speak in baby talk, and concepts like “toddler” and “teenager” may not exist. Researchers have even found that infant personality and temperament vary around the globe. Compared with Dutch babies, American babies are grouchier and harder to soothe, which no doubt shapes their mothers’ behavior. On the flip side, researchers suspect that American mothers’ collective unhappiness in turn exacerbates their babies’ grumpy temperament.

Cultural and economic variables may also help explain why postpartum depression rates seem to vary globally. When researchers recently analyzed postpartum depression data from 56 countries, they found that income inequality made a difference: Poor but relatively egalitarian countries often had less depression than rich countries with high income inequality. So did the amount of paid work that women of childbearing age on average performed, with more than 40 hours a week correlating with upticks in national depression.

The outside work itself is not necessarily a problem — indeed, moms who have strong social networks at work and derive satisfaction from their jobs often thrive. But writ large, the amount of labor that young mothers are performing may be a sign that material resources and social support for new mothers are in short supply.

A longtime writer for Smithsonian Magazine, New York Times best-selling author Abigail Tucker is the mother of four children ages 10 and under. Her new book, “Mom Genes: Inside the New Science of Our Ancient Maternal Instinct,” is available wherever books are sold.

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