Does stress sharpen your baby’s mind—or dull it? Can you predict your baby’s temperament? Why are babies born in the darker months of the year more likely to be risk-takers? Are bossy, dominant women more likely to have boys, which skinny women have more girls?

On Armin Brott’s Mr. Dad radio show, I talk about these topics from the book — and more.

When our baby was six months old, my husband and I couldn’t walk down the street without stopping to accept a compliment. Baby was something of a local celebrity or a minor deity. Free pizza and donuts, the choicest table in the restaurant, a bracelet, stickers, sunglasses, a spot at the front of the line — these were some of the offerings made by the infant’s admirers. Ten months have now passed, and now she’s a toddler. To me, she’s as cute as ever, and she still receives a lot of kisses and waves from strangers. But, we’ve got to face it — she doesn’t attract the same the solicitous stop-in-the-street simpering that she did in her first year. How much longer will it be before everyone stops thinking she’s so adorable?

This question happens to be the subject of a new study on children’s faces. A trio of psychologists— Lu Zhu Luo, Hong Li, and Kang Lee — in China, and at the University of Toronto, recruited 60 men and women and showed then a large sample of children’s faces ranging from infants to 6-and-and-half-year-olds. The participants were asked to rate each face’s likeability (How much do you like the face?) and attractiveness (How attractive is the face?). The researchers wanted to know if younger kids would be given higher attractiveness ratings than older kids — and, if so, at what age does the cut-off happen from over-the-top adorable to merely cute?

The answer? Yes, it’s as expected. Men and women rated infants as cuter than toddlers, who, in turn are rated as cuter than young children. The big drop-off in cuteness appears to happen somewhere between preschool and kindergarten. The researchers identified it as approximately age 4 ½.

What happens to school-age kids?

Their facial structure changes, according to the Luo and his colleagues. Infants have a special set of features, such as a protruding forehead, a large head, a round face, big eyes, and a small nose or mouth. As a species, we have evolved to be attracted to babyish features and find them adorable. These cues make us feel soft and protective, whether or not we’re biologically related — which increases the likelihood of the baby’s survival. (Indeed, studies have found that infants that have tiny eyes, flat foreheads, and square faces, for instance, are less likely to receive attention.) Pleasingly, our tendency to prefer infantile faces even extends to infant faces of other races.

Facial cranial growth is gradual, as is a child’s independence from constant care-giving. Children’s faces lose some of their universal appeal right around the age that they don’t need it anymore to (merely) survive — somewhere around kindergarten-age. Incidentally, this interval — four to five years — is the same as natural birth spacing — when our foremothers would become pregnant with their next baby.

Not long ago, someone emailed me a new study on chocolate and temperament. The study came from a group of psychologists at North Dakota State University and Gettysburg College, and it involved personality differences between people who liked sweet foods versus bitter, spicy, sour, salty, or umami flavors. The researchers wanted to know, “Is having a sweet tooth related to having a sweeter disposition?”

Here’s a sample of what they asked their hundred or so volunteers.

Candy, caramel, chocolate cake, honey, ice cream, maple syrup, pears, raisins, strawberries, and sugar. How fond were they of these dessert flavors?

The volunteers rated their taste preferences on a scale of 1-10.

Following that, they completed an agreeableness scale. That is, they were asked to indicate the extent to which they behave in ways reflective of high (e.g., “have a soft heart”) versus low (e.g., “insult people”) levels of agreeableness.

Pleasingly, the researchers found a significant correlation. People who loved sweets were likelier to be more agreeable.

“Further proof that chocolate lovers would have sweeter babies!” my sender gushed. There’s logic here: If people with a sweet tooth really have a sweeter disposition then women who love sweets might have more agreeable babies because disposition is heritable. Equally valid, people who have sweeter dispositions would have a gentler parenting style, resulting in babies with sweeter dispositions. Ergo, chocolate lovers have sweeter babies. Natch.

Then the researchers took their study to the next level to see if sweet-toothed types not only test as more agreeable, but act sweeter too.

At the end of the session, participants were told that the study was over and that full participation credit would be awarded, thereby relieving them of any further obligations. However, it was mentioned that a colleague in the English department was collecting data on media preferences and was looking for volunteers.

Interestingly, people who had a sweet tooth were more likely to complete the voluntary survey, even though they weren’t getting any extra credit or compensation.

Such sweethearts!

I wonder: does the mere suggestion of sweet food make people act sweeter? Does sweet food make people less angry and aggressive? The researchers claim that metaphors can be predictive about behavior and personality. Because “sweet” in English applies to both taste and disposition, does the correlation still apply in different languages?

How sweet would that be?

Among the great mysteries — and horrible injustices — of aging is that dementia strikes more women than men. No one knows why exactly.  For years, scientists suspected that the culprit is estrogen. Implicated in memory formation, estrogen levels plummet after women reach menopause.

But here’s the catch.  If low estrogen levels are behind age-related dementia in women, then why did a massive study by the National Institute of Health find that women who had hormone replacement therapy (HRT) were no less likely to experience cognitive decline than women who didn’t have it?

Because not all estrogens are created equal, nor are all brains.  Motherhood may be the missing link.

This comes from an intriguing new study by Cindy Barha and Liisa Galea, neuroscientists at the University of British Columbia’s Brain Research Center.  Barha and Galea knew that the female brain is highly plastic; it literally restructures itself in the course of pregnancy and caring for a baby.  They were curious about whether motherhood might alter the brain in a way that protects against dementia under certain conditions.

One way to explore this is to study middle-aged rats that are genetically identical in every way with the exception of their reproductive lives.  The scientists divided the rats into groups — virgins and mothers — and injected each with a form of estrogen:  estradiol or estrone (the form in HRT).  Later, they looked at the rats’ brain tissue to see whether new cells had formed — a process called neurogenesis — in the hippocampus where memories are formed.  These new cells may reduce the risk of dementia.

It’s interesting, this contest between middle-aged virgins and mothers.  Compared to virgins who had been injected with estrogens (and mothers who hadn’t), the middle-aged mother rats that took the hormone replacement therapy grew significantly more new hippocampal cells.  The mothers — and only the mothers — grew many new neurons when exposed to the estrogens (especially a combination of estrone and 17a-estradiol).  The hormones helped the mothers’ minds to remain malleable.

Why did the mothers have a seeming advantage?  Anther mystery — which Barha and Galea say may be related to an enhanced ability of mothers’ hippocampuses, even late in life, to respond to estrogens.  Such responsiveness may come from hormone exposure during pregnancy or afterward, or enrichment from the experience of mothering.

Interestingly, estradiol’s effect on the brain is mediated by BDNF (brain derived neurotrophic factor levels), which may be higher in mothers than in those who have not given birth. The researchers note that BDNF may help explain an assocation between high levels of estrogen exposure across a lifespan and a decreased risk of cognitive decline and Alzheimer’s disease. Among rats, those who have been mothers have a reduced age-related decline in spatial memory and other cognitive decline.

The scientists conclude:

Therefore previous reproductive experience, which is associated with altered hormone exposure and greater enrichment, can result in higher levels of plasticity in the middle-aged brain and may protect the brain from the deleterious effects of aging in females.

But there’s a very real caveat.  Applied to humans, these findings are still highly speculative. First, the boost would depend on a woman taking hormone replacement therapy, which remains controversial due to its connection with breast cancer.  (Why hasn’t HRT been shown to improve cognition thus far?  It may because a combination of estrone and 17a-estradiol is much more effective than the current regimen of estrone and progestin.)

More importantly, there is no guarantee that new neurons will improve a person’s ability to learn and remember or decrease the risk of cognitive decline.  As Barha and Galhea warn, all that new growth might even lead to a sort of jungle effect if not properly integrated.  There are clearly other factors that determine what properties these new cells will have as they mature.  Do some mothers put their new neurons to use more effectively than others?  No one knows whether or how, but the question is fascinating.  (More research is underway in Barha and Galea’s lab.)

What’s clear is that motherhood results in permanent changes in the brain.  Will we mothers someday benefit cognitively more than childless women from new hormonal therapies? There’s a chance.  Wouldn’t it be marvelous if a cognitive boost later in life would compensate for all the sleepless nights now?

*If you like this blog, click here for previous posts. If you wish, check out my new book,  Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy. 

Today I was thrilled to see a mention of Chocolate Lovers in one of my favorite parenting sites, Urban Baby.

For women trying not to get pregnant, life should be easy. Conception can only happen 12-24 hours after ovulation. Sure, sperm may last as many as 3-4 days in the genital tract, hanging around for the egg to arrive. But you’d think that, to avoid a pregnancy, all you’d have to do is to abstain from sex for 4-5 days around the ovulation window.

That’s what the rhythm method is — a natural form of birth control that relies on abstinence during a woman’s fertile days.

But slips happen even among the most careful practitioners of the rhythm method. Some of this may have to do with women not keeping perfect track of their menstrual cycles or having naturally irregular cycles. The failure rate for rhythm method is 25 percent each year (with a perfect-use the rate is still nearly 10 percent).

This is shockingly high. Why so high?

The hidden reason could be pheromones, chemical signals that subtly influence our behavior with out our knowing. It’s just speculation in the journal Medical Hypotheses, but it’s worth mentioning. The submission suggests that pheromones that men put out in their sweat and saliva may trigger early ovulation in women.  This phenomenon has been observed in other studies, including this one at the Monell Chemical Senses Center. The early release of the egg — in advance of the expected fertile window — obviously increases the chances of fertilization.

The target chemical is androstadienone, a testosterone-related compound.  It’s not only in men’s sweat, but also in their semen and saliva.  Androstadienone works its charm by increasing the amount of luteinizing hormone in women, which thereby triggers ovulation.  Women inhale the chemical in men’s sweat (or absorb it orally or vaginally), whereby it acts on their hypothalamus, the region of the brain that controls hormone secretion.

It’s possible that high-testosterone men — who produce more androstadienone — may be likelier than low-tesosterone men to have an accelerated-egg release effect on their lovers. Their sweat smell alone may do the trick.

As I mentioned in a previous post, there are other properties in semen that may also trigger early ovulation. For instance, seminal fluid contains follicle stimulating hormone (FSH) and luteinizing hormone (LH), which may coax the ovary to release an egg.

Bottom line: there are many events that may throw off a woman’s cycle. We don’t live in a clockwork universe, nor do we have clockwork bodies.

Note: A previous version of this post contained a reference to NFP, the Catholic Church’s form of birth control. NFP techniques such as cervical mucus and basal temperature readings, etc. are much more reliable than the rhythm method.

*If you like this blog, click here for previous posts. If you wish, check out my new book,  Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy. 

Here’s my essay touching on some fun science behind  fatherhood, The Plight of the Pregnant Male, in this weekend’s WSJ review.

For months I’ve had a hunch that having a baby has been better for my husband’s image than mine. I don’t mean his looks. Neither of us has had much time to sleep and shower and pay attention to clothing and hair. What I mean is that I think first impressions favor fathers more than mothers.  Pushing our baby along in the stroller, holding a coffee cup and singing, I fall into an easy stereotype, but my husband doesn’t. Yes, in terms of public image, I believe a man benefits more from being a father than a woman benefits from being a mother.

Am I right?

A couple years ago, Ariane Kemkes, a researcher at the Tholius Institute for Research in Applied Demography in Scottsdale, Arizona, asked the same question. Kemkes wondered what would happen if you took a picture of a parent sitting next to his or child, and impartial judges rate that parent in terms of attractiveness, smarts, success, and so on.  Then, she wondered, what would happen if you crop the child out of the photo and ask a different set of judges to rate them on the same criteria?

Are men and women judged differently when they’re with kids than without?  And if so, do fathers benefit from a bigger boost in their social image than do mothers?

The results are intriguing to us new parents.

Men perceive mothers and fathers differently than women do. Looking at a photo of mother and child, male judges are 2.4 times more likely than female judges to believe that the woman is committed to family. Female judges were more cynical and critical of other women’s maternal commitment (but more interested in meeting them). Surprisingly – and to my relief – both sexes are marginally (1.1 times for men, 1.2 for women) more likely to think a woman looked more attractive with a child than when she was alone.  Men, however, were more ambivalent abot meeting women if they were mothers. A mother was also perceived by judges of both genders as slightly, but not significantly, more faithful, honest, and mature.

But what about her mind?  Here comes the crux of my argument about the drawbacks of motherhood.  If a woman was paired with her child, both male and female judges perceived that woman to be less ambitious than if she was alone.  The presence of a child around a woman reduced the woman’s likelihood of being regarded as ambitious by as much as 30 percent.  The assumption by men is unsurprising, but that the stereotype is held by other women is startling. The results may make one pause on bring-your-child-to-work day.

And now, what about men — what does fatherhood do to their image?

Only good things, as I presumed.

Men with children were perceived by all as being committed to family.  Interestingly, fatherhood was good for a man’s social life.  Men were 1.2 times more interested in meeting fellow a man with a child than the same man without a child. And here’s another perk of fatherhood: A man with a child  is perceived to be  of a higher social status.  This comes from judges of both genders. Fathers are also believed to be more faithful, mature, honest.  They’re also thought to be more generous – a perception not transferred to women with children.

Kemkes sums up the stereotype: females most often associate maternity as conflicting with career and leisured activities, while males emphasize financial sacrifices.  A childless woman is perceived as ambitious and a childless male is perceived as cash-strapped, immature, or having a lower social status.  For men, there is a strong association, explainable in evolutionary terms, between reproductive and financial status that does not exist for women.

Moreover, as Kemkes points out, men who are fathers are perceived as more generous than their childless counterparts because emotionally unstable men are more possessive and monopolize resources – traits not associated with fatherhood. Being a dad makes a man appear more “prosocial”; that is, generous and willing to cooperate.  Men are likelier to want to meet dads than childless men because fatherhood lowers expectations of inter-male competition.  It’s now established that fatherhood is linked to lower testosterone levels, especially in the first months after a baby’s birth.

There’s a lesson in research of this kind.  While parenthood generally boosts the social image of both genders, it still cripples career women more than men.  Although more fathers are taking care of the their kids while the mother goes to work, the stereotype that mothers aren’t as ambitious still hasn’t budged – in part, perhaps, because many workplaces continue to make it difficult to excel in both.  So the disappointing fact remains:  A man who prominently features a photo of himself with his children on his desk at work is doing more for his career than a woman who does the same.

*If you like this blog, click here for previous posts. If you wish, check out my new book — hot off the press! –  Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.

  A nice review for Chocolate Lovers in today’s Boston Globe

Early one evening late in my second trimester of pregnancy, I was standing in the dairy aisle of the grocery store, with one hand on my back and the other over the kicking baby in my distended belly. A young man approached me, initiated a conversation about the World Cup, and, casually, asked me if I’d like watch the game with him that weekend. “You’re pretty!” he whispered. I was shocked.

I wasn’t putting out a sexy vibe. (Not at all.) I had assumed that any male attention I receive in late pregnancy, including that from my husband, would be friendly, not sexual. Why would a man who is not the expectant father think pregnancy is sexy? But then other women told me similar stories about how they got hit on in third trimester. So I decided to look into it, and it turns out that a study on sexual attraction to pregnancy has recently come out.

A team of Swedish and Italian doctors, led by Emmanuele Jannini and Magnus Enquist, recruited nearly 2,200 men who had joined online fetish groups such as alt.sex.fetish and alt.sex.fetish.breastmilk. They presented a questionnaire that asked the respondents questions about their preferences for pregnant and lactating women. The survey also asked for the sex and age of each sibling, and whether the sibling is a full sibling or not (half-sibling or adopted child). Most respondents reported both a pregnancy and a lactation preference. The average age at which respondents became aware of their preference was about 18 years.

What Jannini and Enquist and their colleagues were searching for was evidence that there was something special about the upbringing of men that are secually aroused by pregnancy. They knew that a specific stimulus early in life can elicit sexual behavior when that animal reaches sexual maturity. For instance, goats that are raised by sheep are sexually aroused by sheep only. This is called sexual imprinting.

Is it possible that boys that are raised by women who are pregnant for much of their childhoods are unusually attracted to pregnant women?

It turns out, what’s good for the goat is good for the guy. The more exposed a man was to his mother being pregnant and breastfeeding when he was between 1.5 and 5 years old, the more likely he is, as an adult, to be sexually attracted to pregnant and breastfeeding women.

A younger sibling is the key to early exposure. The respondents who eroticized pregnancy and breastfeeding had significantly more younger siblings than expected by chance. Respondents with one sibling were older than their sister or brother in 66 percent of cases. Interestingly, siblings born of a different mother does not appear to be related to respondents’ sexual preferences. Only a boy’s own pregnant mother seemed to leave a sexual imprint.

Freud’s “oedipal phase,” from about 3 to about 5-6 years of age, only overlaps partially with the sensitive period suggested by this study’s data, the researchers are careful to point out. Sexual imprinting is different in that it’s motivated not by sexual drive but because the individual learns what’s normal during a sensitive phase of development and later seeks sexual partners that resemble his (or her) own parents.

What does this mean for women who are pregnant or plan to be pregnant? It means you may be able to predict how attracted your partner will be to you in late pregnancy. Does he have sibling born within five years after him? If so, he’s likelier to be turned on by your pregnant self.

As for the guy I met in the dairy aisle, I’d wager he had a younger brother or sister. I’d bet more on getting this right than the winner of the next World Cup.

 *If you like this blog, click here for previous posts and here to read a description of my most recent book, Do Gentlemen Really Prefer Blondes?, on the science behind love, sex, and attraction. If you wish, check out my forthcoming book, available October 11,  Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.