The Biology of a Mother’s Love

Mother-infant bonds predicted by hormone levels in humans and other mammals

Image: "Mother's Love" by Kolongi Brathwaite
A common tactic by evolution deniers is to claim that if a complex behavior can’t be measured than the scientific method must be a flawed approach towards understanding the world. Nevermind that no one challenges the science of physics just because we can’t predict the complex motions of a leaf in a windstorm. But when it comes to matters of emotion somehow natural explanations are off limits. This is readily apparent in the common argument that, “if you think biology is such a good explanation of behavior, then prove that your mother loves you.” However, as it turns out, we can address this challenge of motherly love and demonstrate a plausible scientific explanation by measuring the levels of the important hormones involved.

Writing in the current issue of Psychological Science (subscription required), Ruth Feldman and colleagues at the Gonda Brain Research Center at Bar-Ilan University in Israel have found evidence that neuroendocrine levels of the hormone oxytocin is a strong predictor of a mother’s bond with her infant. By sampling the blood oxytocin levels of 62 pregnant women (of all educational and employment backgrounds) the researchers found that oxytocin levels remained consistent throughout their pregnancy but differed substantially between the women. By then analyzing video footage of the mothers’ interactions with their infants (which included analysis of how often they gazed at the infant’s face, their amount of affectionate touching, rocking, and how often they spoke in motherese to their child) the researchers found that levels of oxytocin was the major factor in predicting the levels of maternal bonding.

As the authors reported in their study:

The results suggest that the neuroendocrine system associated with bond formation in mammals may play a similar role in humans. OT [oxytocin] was found to be related to a well-defined cluster of maternal behaviors, attachment representations, and a specific maternal behavior that appears across mammalian species . . . These findings lend support to ethological and evolutionary perspectives on human bonding.

Macaque mother with nursing infant.

Image: Paula Bronstein/Getty Images
In other words, for all mammals there’s evidence that high levels of oxytocin translate into a feeling of personal attachment with their infant. Earlier studies on oxytocin have shown that the hormone is also involved in pair-bonding and cooperative behavior. For example in the closely related prairie and meadow voles, the former is a pair-bonded species that shows high levels of both maternal and paternal care while the latter are neither pair-bonded nor attentive to their offspring. Work carried out by Thomas Insel at Emory University has shown that oxytocin receptor density is the primary difference between the two species.

As to why some individual’s have high oxytocin levels and others don’t is still an open question. Research on primates and rats has shown that daughters who grew up feeling safe and secure with high levels of parental investment demonstrated the same parental behavior with their offspring. It’s likely that a safe and nurturing environment (with both economic and social support for the mother) would increase a mother’s oxytocin levels and could thereby increase the amount of maternal bonding.

However, it’s important to point out that there is not one “optimal” maternal behavior for all environments. Human mothers respond to their surroundings in the same way that other species do. Whether you’re a mouse living in desert landscape or a woman in an impoverished city center, if an environment is particularly harsh it may well be more adaptive for mothers to show less maternal bonding and thereby raise an infant who will be hardened for a difficult life. It’s also important to point out that while these results are highly significant, there is more than just chemistry that influences a mother’s love. We shouldn’t underemphasize the personal decisions or the cultural influences that a woman encounters that influence her maternal behavior. To do so would be to miss the larger picture and not show our full appreciation for the sacrifices that mothers make.

While many interactions are likely to be involved in maternal behavior (both hormonal and social), this study shows that a mother’s love can be partly quantified and predicted using the tools of the scientific method. More than a final retort to my hypothetical interrogator, what this study shows is how remarkably conservative and elegant the products of natural selection can be. To think that a single hormone is identical across mammalian species and can influence one of the most profound feelings imaginable is an awe inspiring thought. To cop a quote from Darwin himself, there truly is “grandeur in this view of life.”

Reference:

Ruth Feldman, Aron Weller, Orna Zagoory-Sharon and Ari Levine (2007). Evidence for a neuroendocrinological foundation of human affiliation: plasma oxytocin levels across pregnancy and the postpartum period predict mother-infant bonding. Psychological Science 18(11):965-970. doi:10.1111/j.1467-9280.2007.02010.x

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Eye of the Beholder

Psychologists determine that eyes are the window to a woman’s desire.



As a species we are consumed by love. Ask yourself, how many cultural productions (films, stories, songs, dances, arts) do not have love, the loss of love or the absence of love as their central theme? Would you be satisfied with what is left over? Love’s power over us is just one reason why evolutionary research is so fascinating.

A well-worn trope of human culture is men’s obsession with female infidelity. Othello. Madame Bovary. Desperate Housewives. These are just three Western examples of this concern that are paralleled in every society and throughout time. Such powerful and universal human emotions suggest a biological commonality that can be revealed through a scientific lens.

Past research has demonstrated that women alter their sexual preferences based on where they are in their menstrual cycle. Women tend to be much more prone to initiate sexual encounters during their ovulatory period when sex could result in reproduction. Further, in studies based on modern Western societies, women are more likely to fantasize about having sex (as well as make those fantasies reality) with a man other than their current partner during this most fertile phase of their cycle.

Now, a study published online today in Hormones and Behavior demonstrates that this increased sexual proceptivity is apparent simply by looking into a woman’s eyes. Using 14 women (7 on birth control and 7 not) Bruno Laeng and Liv Falkenberg from the Department of Psychology at the University of Tromsø, Norway have shown that women’s pupils dilate more widely to photos of men they were sexually attracted to during their period of greatest fertility.

The researchers had the women look at a computer screen during three different periods of their menstrual cycles to view photos of individuals of sexual significance (their boyfriend or a favorite actor) as well as individuals of unknown sexual significance (their favorite actress, a Norwegian weatherman or an elderly politician). Each time the photos appeared on the screen the women’s pupil size was recorded using an infrared eye-tracking device. The authors determined that:

The findings confirmed the presence of cyclic differences in papillary diameters while watching facial portraits of sexually interesting individuals. Remarkably, the participants using contraceptive pills did not show cyclic fluctuations of pupil sizes.

The results suggest that using birth control flattens the responses by releasing higher levels of progesterone and suppressing estrogens during the fertile period. Also, testosterone, which peaks at midcycle and is directly related to sexual motivation, is suppressed in women on the pill and could negatively influence women’s sexual desire (indeed, reduced libido is one of the common side effects of taking birth control).

Of the subjects not on birth control, the study found that women’s pupils grew largest during their fertile period when viewing pictures of their boyfriend. A close second was the women’s favorite actor.

Assuming that a favorite actor represents the most likely candidate for an extra-pair sexual encounter, an increased response to such a target of interest would be entirely consistent with previous reports that women (at least, in Western societies) report to be more prone to fantasize or have sexual intercourse with a man other than their current partner during the fertile phase of the cycle.

From an evolutionary perspective this is sometimes represented as the selective advantage between dads and cads. During most of their cycle women tend to be most attracted to men who demonstrate good paternal characteristics --- tender, patient, attentive, loyal --- qualities that are often present in individuals with lower levels of testosterone. However, during their peak of ovulation, women on average are more attracted to muscular and/or assertive men who are statistically more likely to make poor long-term partners. Not coincidentally, these are also individuals with higher levels of testosterone.

The arms race between male and female sexual strategy is one of maximizing individual reproductive success. It should come as no surprise that these strategies sometimes differ. For women the strategies are to either choose a partner with phenotypes indicative of high status in order to directly benefit their child or to choose a partner who will assist in child-rearing to ensure that child’s survival. However, as primatologist Sarah Hrdy has suggested with what she terms “Female Plan B,” why not have both? Choose a man who will be a devoted partner and assist in childrearing, but then have a child with a different man who has the genetic qualities promoting high status. If the woman can get away with it, the dad will then help to raise the cad’s bastard (there was a time when that term had serious negative connotations for any children labeled as such).

This has been demonstrated in gibbons, a monogamous “lesser ape” native to Asia and the Indian subcontinent. In a study published in the journal Ethology, 12% of the otherwise monogamous females engaged in extra-pair copulations, or matings with individuals who were not their partners. This is somewhat lower than the current statistics for Western women, of whom 26 to 70 percent engage in extra-pair copulations. The differences aren’t so much important as is the fact that this is a common strategy in females of both species. Females, like males, will use flexible strategies to ensure their reproductive success, regardless of the negative stereotypes our culture places on one rather than the other.

So that flicker of delight you see in your girlfriend’s eyes while watching Ocean’s 13, the slight dilation of her pupils that seems a little bit larger than usual during a certain time of the month, that’s our evolutionary legacy playing itself out today in our living rooms. But don’t panic guys, according to Laeng and Falkenberg’s study women are still mostly interested in the man they’re with. Just make sure you continue to earn that interest.

References:

Laeng, B. and Falkenberg, L. (2007). Women’s pupillary responses to sexually significant others during the hormonal cycle. Hormones and Behavior. doi: 10.1016/j.yhbeh.2007.07.013

Reichard, U. (1995) Extra-pair copulations in a monogamous gibbon (Hylobates lar). Ethology 100(2):99-112

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Defeating Your Inner Rat

If you're like me, then you often feel like an anthropologist on Mars. I observe in confusion as men around me (who apparently share the same sex characteristics I do) behave in ways that would make the war god blush.

I watch as males of our species bluff and strut in what they call friendship. They smack each other around. They downplay one another's achievements while exaggerating their own. They refer to each other using derogatory terms for women's anatomy or as females of the order canine. What a bunch of creeps!

Even worse, I find myself struggling to resist being like them. It's as though I'm in a 1950s werewolf movie or suffering from Tourette's syndrome. I'm desperately trying to keep the monstrous behavior from erupting once again.

As a kid I participated in such masochistic enterprises as hitting your best friend in the arm until he admits pain. When a group of us got together we'd play violent and competitive games like King of the Mountain or Basketbrawl or Monopoly. I would even watch professional wrestling and cheer as two testosterone-poisoned monsters pretended to bash each other's heads in. "Boys will be boys," is the excuse I've usually heard.

This was the conclusion of a paper presented at the American Physiological Society Conference on Oct. 18. The paper, "Sex differences in brain monoamines and aggression," by Jonathan Toot, Gail Dunphy and Daniel Ely of the University of Akron, stated the traditional maxim with some jazzed-up terminology: "Aggression is influenced by a Y-chromosomal effect that decreases amygdala 5HT." In other words, males show more aggression because our Y-chromosome codes for lower amounts of the neurotransmitter serotonin in the part of the brain called the amygdala. When the amygdala is stimulated, serotonin decreases, testosterone increases and aggression results. Simple, right? Forget special child rearing. Get out the pruning shears.

It would appear that Toot, Dunphy and Ely have solved the riddle about why boys are such pains. However, there is another consideration. Their study was done on rats. This isn't as big of a deal as you might think. The evolution of mammals has left us all with the exact same neurotransmitters and pretty similar brain structures. That's why we torture other members of our class in labs around the world to research drugs and behavior. The important consideration is simply this: Rats lack the long period of childhood dependency that humans have.

Humans come out of the oven pretty undercooked. It's at least 12 to 13 years before we're sexually mature, and up to 20 years before we're physically mature. During this period, we rely on adults for food, protection and as models for our behavior. In our current world, these models are extended by the mass media - which overwhelmingly provide entertainment that appeals to our more socially damaging instincts. Hundreds of studies have shown that violent individuals usually come from violent environments. It's reasonable to assume that male aggression in humans is strongly influenced by the environment as well.

What we share with our evolutionary cousin the rat is the innate tendency towards aggression, but biology is not destiny. What you encounter in life, especially early in life, can dramatically affect how you live. Males may have the potential for more aggression, but if it's not emphasized, perhaps it won't be expressed. After all, my father is a very gentle and loving man. Maybe this has something to do with why I'm curious to understand aggression, but don't take part in it anymore (the occasional kung-fu flick notwithstanding).

This is a powerful and world-changing idea, since it's men who presently control most governments on our fragile Earth. Of course, ritual castration of public servants is still a viable option.

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