Committed Relationship: You’re Wired For It

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Pair bonding is a biological program not a cultural construct

Porn addiction can interfere with relationshipsDespite a colorful array of cultural differences, humans everywhere fall in love, attach emotionally for long periods, and feel betrayed when mates are unfaithful. These behaviors are innate, not the products of random cultural influences. To make this point another way: Most mammals don't tattoo their mates' names on their bums, and are not subject to fits of jealous rage.

Human Brains Are Built to Fall in Love, an earlier post, explained that pair bonding behaviors have neurobiological mechanisms behind them. Now, there's more research evidence of our underlying pair-bonding programming. Predictably, it lines up with the evidence coming from the famous pair-bonding prairie vole. (More on that in a moment.) New York Times journalist John Tierney describes the new research this way:

The 21-year-old woman was carefully trained not to flirt with anyone who came into the laboratory over the course of several months. She kept eye contact and conversation to a minimum. She never used makeup or perfume, kept her hair in a simple ponytail, and always wore jeans and a plain T-shirt. ...

Previous research had shown that a woman at the fertile stage of her menstrual cycle seems more attractive, and that same effect was observed here—but only when this woman was rated by a man who wasn't already involved with someone else.

The other guys, the ones in romantic relationships, rated her as significantly less attractive when she was at the peak stage of fertility, presumably because at some level they sensed she then posed the greatest threat to their long-term relationships. To avoid being enticed to stray, they apparently told themselves she wasn't all that hot anyway. ...

Tierney adds that clearly,

Natural selection favored those who stayed together long enough to raise children: the men and women who could sustain a relationship by keeping their partners happy. They would have benefited from the virtue to remain faithful, or at least the willingness to appear faithful while cheating discreetly.

He also quotes U.C.L.A. psychologist Martie Haselton: "Women and men are affected by ovulation, but we [humans] don't have any idea that it is what is driving these substantial changes in our behavior. [Such research] makes it clear that we're much more like other mammals than we thought."

How true. In voles, scientists are already uncovering the underlying neural mechanisms that regulate pair bonding behaviors, and sure enough, one of them is a mechanism that causes a male's defensive aggressiveness toward unfamiliar willing females (once he has formed a pair bond with his main squeeze). This behavior obviously doesn't happen for cultural reasons. It happens primarily because the neurochemical vasopressin increases in a key part of his brain. (By the way, it won't necessarily keep Mr. Vole 100% faithful. Mrs. Vole, too, has been known to have a fling.)

Curious about the mechanics that dictate whether a mammal is capable of bonding? Turns out that in monogamous voles natural selection has reconfigured the distribution of oxytocin and vasopressin receptors in the brain's limbic system. While all voles find sex rewarding, monogamous voles also get good feelings from a particular mate. The bonding mechanism, by the way, is the original addiction mechanism (which all other addictions hijack). This is why addiction can interfere with pair bonds.

Indeed, if scientists trigger the production of too much dopamine with artificial stimulation, an animal not only doesn't bond, but also becomes aggressive toward all females. Could this help explain why some heavy porn users appear to be losing interest in real mates?

The vital point is that our pair bonding penchant arises from physiological events, not mere social conditioning. It evolved from the infant-caregiver mechanism, and the two mechanisms still overlap in the brain's reward circuitry. So, even though many Westerners appear to be caught up in a chaotic hook-up culture for the moment, it doesn't mean that we humans are, by nature, as promiscuous as bonobo chimps or that pair-bonding inclinations are superficial cultural constructs.

Keep in mind that human and bonobo evolution diverged some six million years ago. Our closest relatives are on our branch of the evolutionary tree, even if they're no longer around. Somewhere along that branch humans became pair-bonders due to brain changes.

Although mammalian pair bonding is rare, the alterations that make a species pair bonders are not necessarily exotic. For example, pair-bonding prairie voles are so like their promiscuous meadow-vole cousins that scientists can convert a meadow vole into a pair bonder simply by prompting the expression of a single gene in his forebrain. (It increases receptors for vasopressin). In short, the behavior of our distant bonobo cousins is entertaining, but quite irrelevant when it comes to understanding the fundamentals of human mating.

"Ah, but look how promiscuous we are!" you're thinking, right? Keep in mind two other points about our current hook-up behavior:

First, the studies we do in the West (generally using university students) are somewhat reckless in characterizing all of human behavior based on small slices of our rather unrepresentative culture. While strict monogamy is not the human norm, most mates still live in pairs. (Many cultures permit a man who can afford it to take another wife, but few can afford it.)

In short, if you are hooking up with multiple partners purely for recreation you could be an outlier. Your behavior is not typical human behavior—a point that is easily overlooked by Western researchers. For example, a 2007 study of 1,500 undergraduate men and women claimed to tell us "Why Humans Have Sex." It found that many students had sex for recreation, not procreation. (Really??) In other cultures, humans are often quite adamant that sex is primarily linked to reproduction and building a family. Even sex with multiple partners may have stronger babies ("seminal nurture") as its objective. (Lest readers jump to any misguided conclusions, I'm a 'sex for recreation' fan, but also a fan of the benefits of attachment.)

Second, the term "pair bonder" does not imply perfect sexual monogamy. It simply means mates are inclined to hang out together and raise offspring (known as social monogamy). No pair-bonding mammal species are entirely sexually exclusive; it would be an evolutionary handicap. So the reality that not all humans remain one hundred percent faithful for life and some of us have sex without attachment is not surprising. Variety also serves evolution.

Yet it's useful to keep in mind that pair-bonder brains, including yours, are generally set up to attach to a mate. So, even if your milieu is wildly promiscuous for the moment, you have nothing to apologize for if you notice a hankering for a stable bond at the center of your sex life. The reasons lie in your brain, not your upbringing, and you can consciously tap this innate potential.

In our culture, which so prizes beauty and youth, it may seem downright odd that an aging couple could be more and more pleased by each other as the years pass. ... If you know a handful of elderly couples, think about those among them who are still intensely drawn to one another. Watching them is evidence enough that attraction is not primarily based on attractiveness. ... Seeing, touching, and hearing a devoted partner gains more and more power over time to trigger the release of [the bonding hormone, oxytocin].—Mark Chamberlain PhD

At least in a pair-bonding species like us.

See "The Ape That Thought It Was a Peacock:Does Evolutionary Psychology Exaggerate Human Sex Differences?"

(Excerpt) Pair Bonding

Pair bonding (or monogamy) is an extremely rare mating system among mammals, found in less than 5% of species (Kleiman, 1977). Nonetheless, it appears tobe a central element in humans’ reproductive repertoire. It is therefore a curious fact that our dominant mating system is more like the typical mating system of birds than that of most mammals, including our nearest relatives, the Great Apes. In making this claim, it is important to be clear about three things. First, the claim is not that pair bonds necessarily last for life. In the absence of socially  enforced lifelong monogamy, most pair bonds last for months or years but ultimately dissolve (Fisher, 1992). Note, though, that a significant minority of pair bonds do last until the end of the lifespan, even in traditional forager societies that lack rigid strictures on divorce (see,  e.g., Marlowe, 2004).
Second, the claim is not that human pair bonds are always sexually exclusive. Most surveys suggest that considerably fewer than 50% of men or women in long-term committed relationships are ever unfaithful (Blow & Hartnett, 2005). Nonetheless, some are, and as a result, a certain fraction of offspring are sired by someone other than the social father (the best estimates place this at around 1–3%; Anderson, 2006; Wolf, Musch, Enczmann, & Fischer, 2012). Third, the claim is not that pair bonding is our one “true” or natural mating system. Humans exhibit all the mating systems found in other species, including monogamy, polygyny (one man, two or more women), and even polyandry (one women, two or more men; Murdock, 1967).
It is also not uncommon for people to engage in extrapair mating, or to engage in casual sex before marriage or between long-term relationships. Different frequencies of each of these mating behaviors are found in different cultures and different historical periods. However, with the exception of long-term polyandry, all are relatively common, and thus all are plausibly part of the evolved repertoire of the human animal. Thus, our claim is not that pair bonding is humanity’s singular mating pattern. Our claim instead is simply that the pair bond is the most common setting for sex and reproduction in our species, that it has been for a long time, and that this has left a deep imprint on our evolved nature.

2016 study: Prairie voles show human-like consoling [but non-pair-bonding voles do not]




Our forefathers were fierce & our foremothers were faithful

By Razib Khan | October 27, 2013 8:51 pm

Credit: ChineebOne of the peculiarities of human historical genetics is that people can simultaneously accept the existence of aggressive polygynous males such as Genghis Khan, and promiscuous females who give rise to the idea that 1 out of 10 children have an incorrect assigned paternity. I’ve mentioned the cuckoldry myth before. It is a common evolutionary myth; I’ve heard many biologists quote the 1 out of 10 number, and have often made myself obnoxious by pointing to the contradictory literature in this area. This isn’t to say that cuckoldry doesn’t exist. There’s certainly an evolutionary reason so many males engage in “mate guarding.” But you don’t need a high frequency of a trait to allow it to be selectively constrained. If it’s deleterious, then it will be driven down in frequency rather quickly. Whenever you get outbreaks of males who are sanguine about providing resources for offspring who are not their biological issue, natural selection will kick in and guarantee that this generous spirit toward their cheating partners and the delinquent cads does not persist.

The way that modern genetics adds value to this area is that one can compare Y chromosomal lineages to surnames. The logic is simple. If you have a constant frequency of misattributed paternity per generation over time the correlation between a surname and a Y chromosomal lineage will weaken. In addition, because the interlopers are likely to be different from each other you’ll have a pattern with (for example) ~50% of the male individuals of a given surname may carry one haplotype, while the other ~50% are distributed across hundreds of other haplotypes (one can imagine twists on the scenario, for example an early interloper might result in a secondary highly frequent haplotype).

So here’s an new study to bury this tired old urban myth, Low historical rates of cuckoldry in a Western European human population traced by Y-chromosome and genealogical data:

Overall, our results provide the first large-scale, unbiased genetic study of historical EPP rates in a human Western European population, with two independent estimation methods giving largely concordant results. Using the most direct estimation method, based on pairs of males that had a GCA in the last few centuries, we estimated the average EPP rate at 0.91% per generation (95% CI: lower bound 0.41% and upper bound 1.75%). This method took advantage of the hypervariability and mutability of Y-STR haplotypes, and the high phylogenetic resolution of the used Y-SNP haplogroups, which allowed paternally unrelated males to be easily recognized as such [35]. In addition, using a second method that was based on the population genetic traces of a past immigration event which happened at the end of the sixteenth century, we estimated the EPP rate at around 2%. Although this estimate had a broader CI (upper 95% confidence limit = 8%), the actual estimate was close to the first one.

Both of our methods therefore estimated a substantially lower historical EPP rate for Flanders than the 8–30% per generation suggested by previous studies based on behavioural data on rates of EPCs in Western Europe and given the absence of reliable contraceptive methods [30–33]….

It’s open access, so read the whole thing if you aren’t convinced.

The authors are clear that this is from a sample in Flanders, but I do not find that this population should be that exceptional across the Eurasian Ecumene. In other words, I’ll be willing to put down money that Indian gotras and Chinese patrlineal clans will exhibit the same pattern of cuckoldry frequency. Additionally, the authors note that this pattern of high paternity confidence is paired with male investment in offspring. That seems relatively typical among many members of our species, though the extent seems to vary by population and environmental condition.

How the penis lost its spikes

Humans ditched DNA to evolve smooth penises and bigger brains.

Sex would be a very different proposition for humans if — like some animals including chimpanzees, macaques and mice — men had penises studded with small, hard spines.

Now researchers at Stanford University in California have found a molecular mechanism for how the human penis could have evolved to be so distinctly spine-free. They have pinpointed it as the loss of a particular chunk of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling.

"It is a small but fascinating part of a bigger picture about the evolution of human-specific traits," said Gill Bejerano, a developmental biologist at Stanford who led the work along with colleague David Kingsley. "We add a molecular perspective to a discussion that has been going on for several decades at least."

Published in Nature today1, the research also suggests a molecular mechanism for how we evolved bigger brains than chimpanzees and lost the small sensory whiskers that the apes — who are amongst our closest relatives and with whom it has been estimated we share 96% of our DNA — have on their face.

Monogamous strategy

It has long been believed that humans evolved smooth penises as a result of adopting a more monogamous reproductive strategy than their early human ancestors. Those ancestors may have used penile spines to remove the sperm of competitors when they mated with females. However, exactly how this change came about is not known.

The researchers did not set out to study penile spines. Rather, they were looking for chunks of DNA that had been lost from the human genome but not the chimp genome, so they could then try to pinpoint what those chunks did.

The approach differs from that in most studies, explain Bejerano and Kingsley, in looking at what has been deleted from the human genome rather than what is present. "In the case of our study, had you started from the human genome, there would be nothing there to see," says Bejerano.

They first systematically identified 510 DNA sequences missing in humans and present in chimps, finding that those sequences were almost exclusively from the non-coding regions of the genome, between genes. They then homed in on two sequences whose absence in humans they thought might be interesting — one from near the androgen receptor (AR) gene and one from near a gene involved in tumour suppression (GADD45G).

Inserting the chimpanzee sequences into mouse embryos revealed that the former sequence produced both the hard penile spines and sensory whiskers present in some animals. The latter sequence acted as a kind of brake on the growth of specific brain regions — with the removal of its function appearing to have paved the way for the evolution of the larger human brain.

"The goal of the project was to find molecular lesions [losses] that underlie human evolutionary traits, with the examples illustrating different aspects of the principle," says Kingsley.

"Until we looked at where the DNA was expressed, we had no idea which switch — if any — it would actually control," adds Bejerano.

Other molecular biologists praised the work for its clever approach and said it would open up new avenues of inquiry, particularly for those working on the evolution of the human brain.

"It is detective work and a great reminder that, in the course of evolution, information is both gained and lost," said Sean Carroll, an expert in animal genetics and evolution at the University of Wisconsin, Madison.

"As so often with very good ideas, it seems almost obvious in hindsight," said Svante Pääbo, who directs the genetics department of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and was part of the team that recently sequenced the Neanderthal genome. "Since two of the almost 500 deleted sequences they identified turn out to be interesting, I am sure that several other ones on their list will turn out to be interesting too," he added. The researchers are continuing to analyse the remaining 508 DNA sequences.

David Haussler, who studies the molecular evolution of the human genome at the University of California, Santa Cruz, added that our ancestors' loss of penile spines is our gain today."Couples everywhere can be thankful that this particular piece of DNA was ditched," he says. 

  • References
    1. McLean, C. Y. et al. Nature 471, 216-219 (2011).

Men with smaller testes than others are more likely to be involved in hands-on care of their toddlers, a new study conducted by anthropologists at Emory University finds. The Proceedings of the National Academy of Sciences (PNAS) published the results of the study Sept. 9.

Smaller testicular volumes also correlate with more nurturing-related brain activity in fathers as they are looking at photos of their own children, the study shows. “Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort,” says Emory anthropologist James Rilling, whose lab conducted the research.

The goal of the research is to determine why some fathers invest more energy in parenting than others. “It’s an important question,” Rilling says, “because previous studies have shown that children with more involved fathers have better social, psychological and educational outcomes.”

Evolutionary Life History Theory posits that evolution optimizes the allocation of resources toward either mating or parenting so as to maximize fitness. “Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort,” says Jennifer Mascaro, who led the study as a post-doctoral fellow in the Rilling lab.

While many economic, social and cultural factors likely influence a father’s level of caregiving, the researchers wanted to investigate possible biological links.

They knew that lower levels of testosterone in men have been correlated with greater paternal involvement, and that higher levels of the hormone predict divorce as well as polygamy.

The testes, in addition to producing testosterone in males, also produce sperm. “Testes volume is more highly correlated with sperm count and quality than with testosterone levels,” Mascaro says.

The study included 70 biological fathers who had a child between the ages of 1 and 2, and who were living with the child and its biological mother.

The mothers and fathers were interviewed separately about the father’s involvement in hands-on childcare, including tasks such as changing diapers, feeding and bathing a child, staying home to care for a sick child or taking the child to doctor visits.

The men’s testosterone levels were measured, and they underwent functional magnetic resonance imaging (fMRI) to measure brain activity as they viewed photos of their own child with happy, sad and neutral expressions, and similar photos of an unknown child and an unknown adult. Then, structural MRI was used to measure testicular volume.


The findings showed that both testosterone levels and testes size were inversely correlated with the amount of direct paternal caregiving reported by the parents in the study.

And the father’s testes volume also correlated with activity in the ventral tegmental area (VTA), a part of the brain system associated with reward and parental motivation. “The men with smaller testes were activating this brain region to a greater extent when looking at photos of their own child,” Mascaro says.

While testosterone levels may be more related to pre-copulatory, intrasexual competition, testicular volume may reflect post-copulatory mating investment, the researchers theorize.

Although statistically significant, the correlation between testes size and caregiving was not perfect.

“The fact that we found this variance suggests personal choice,” Rilling says. “Even though some men may be built differently, perhaps they are willing themselves to be more hands-on fathers. It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them.”

A key question raised by the study findings is the direction of casualty. “We’re assuming that testes size drives how involved the fathers are,” Rilling says, “but it could also be that when men become more involved as caregivers, their testes shrink. Environmental influences can change biology. We know, for instance, that testosterone levels go down when men become involved fathers.”

Another important question is whether childhood environment can affect testes size. “Some research has shown that boys who experience childhood stress shift their life strategies,” Rilling says. “Or perhaps fatherless boys react to the absence of their father by adopting a strategy emphasizing mating effort at the expense of parenting effort.”

The study focused only on direct paternal care, and not indirect forms of care, such as protecting children and earning a living to provide for them.

In the decades since the 1960s, the number of women raising children on their own in the United States has risen dramatically. “Although there are more households with no fathers, when the fathers are around, they tend to be much more involved than in previous decades,” Mascaro says.

Much of the existing scientific literature on nurturing is focused on mothers, Rilling notes. “Mothers definitely have more of an impact on child development, but fathers are also important and their role is understudied.”



Coupling up might have been the best move our ancestors ever made

Mammals are not big on monogamy. In fewer than 10 percent of species is it common for two individuals to mate exclusively. The primate wing of the group is only slightly more prone to pairing off. Although 15 to 29 percent of primate species favor living together as couples, far fewer commit to monogamy as humans know it—an exclusive sexual partnership between two individuals.

Humans obviously have an imperfect track record. People have affairs, get divorced and, in some cultures, marry multiple mates. In fact, polygamy appears in most of the world's societies. Yet even where polygamy is permitted, it is the minority arrangement. Most human societies are organized around the assumption that a large fraction of the population will pair off into enduring, sexually exclusive couples. And monogamy seems to have done our species good. “Pair bonds,” as scientists call monogamous relationships, were a crucial adaptation that arose in an archaic forebear that became central to human social systems and our evolutionary success. “We have a very big advantage over many other species by having pair bonds,” says University of Montreal anthropologist Bernard Chapais. ...