Making Sense of Biology
Nothing in Biology Makes Sense Except in the Light of Evolution, Theodosius Dobzhansky (1973). The American Biology Teacher, 35(3), 125-129.
“Love is a smoke…”
No topic can be more apropos in February than romantic love. The reader may take offense immediately. Obviously anyone who claims that they can “make sense” of love, must be either delusional or disingenuous. Fear not my readers; for I am neither of those things.
Love has been the subject of countless stories and poems. Shakespeare describes it thusly:
“Love is a smoke raised with the fumes of sighs;
Being purged, a fire sparkling in lovers’ eyes;
Being vex’d, a sea nourish’d with lovers’ tears:
What is it else? A madness most discreet,
A choking gall and persevering sweet.
Romeo and Juliet, II, 1.
Until the mid-19th century, it is fair to say that while experienced by millions, there could not have been a fully scientific understanding of the function and mechanisms producing romantic love. The Apostle Paul disparaged it, claiming that it was a distraction from an individual’s duty to serve the lord1. Yet despite Paul’s admonishments, we know that passionate, romantic love is found in all cultures2. This suggests that the brain’s capacity for this is ancient. If we view romantic love as part of the continuum of intimacy and sexual gratification, then the link to reproduction can be easily seen. Once a link to reproduction is agreed upon, then it also follows that the process of intimacy, sexual gratification, romantic love must have been under natural and sexual selection.
Evidence that pair bonding has evolutionary significance is demonstrated by its widespread occurrence in species that provide parental care to their young. Indeed, it has been argued that pair-bonding is an important adaptation that facilitates certainty of paternity for males in a variety of species. For fertilization to occur, insemination and ovulation must occur at the same time. It turns out that the same hormonal changes, such as alterations of estrogen and progesterone levels which regulate the maturation of an egg also play crucial roles in enhancing female sexual receptivity3.
In the classic work, The History of Human Marriage, Westermarck describes how multiple legends on the origin of human marriage relate to regulating promiscuity4. For example in the Mahabharata (from India): “women were unconfined, and roved about at their pleasure, independent. Though, in their youthful innocence, they went astray from their husbands, they were guilty of no offense for such was the rule in early times.” But Swetaketu, son of the Rishi Uddalaka, could not bear this custom, and established the rule that thenceforward wives should remain faithful to their husbands and husbands faithful to their wives. He also gives examples from Chinese culture: “…in the beginning men differed in nothing from other animals in their way of life. As they wandered up and down the in the woods, and women were in common, it happened that children never knew their fathers, but only their mothers. The emperor Fou-hi abolished, however this indiscriminate intercourse of the sexes and instituted marriage.” Similar stories are recounted from the Greeks and the Egyptians. Paul’s first letter to the Corinthians, addresses what he considered the immorality amongst the Greeks (Gentiles) and he made the same statement as Swetaketu and Fou-hi5.
These accounts illustrate why males should be interested in formal pair-bonding rituals (marriage) in human societies. Yet they do not make it as clear why females should be as interested. However, the evolutionary theory of human mate preference provides us with more clues.
The theory suggests that mate preferences would evolve if individuals received reproductive advantages over individuals who showed no such preferences and thus mated randomly. For example, this has been demonstrated in many non-human species, such as the African Village weaverbird. Males attract females by displaying his recently built nest. He suspends himself upside down and vigorously flaps his wings to gain her attention. If the female is impressed by the flapping display she will enter the nest and inspect it, she prods, pokes, and pulls at the materials. If the nest does not meet her standards she departs and will inspect another male’s nest. The idea here is the female weaverbird is evaluating this mate’s potential to help her successfully rear her offspring (a poor nest may not shelter, leaving the young vulnerable to the elements and predators.)
There is no reason to assume that our ancestors did not face the same issues. Thus, if human females were to be successful they also had to solve mate preference correctly. Due to the time it takes to rear human children, our female ancestors would have had to:
1. Choose a man who would commit to a long-term relationship.
2. Women who choose a mate who was flighty, impulsive, philandering, or unable to sustain relationships found themselves raising their children without a mate, and therefore might have been at a disadvantage with regard to material resources, aid, and protection.
If we are to believe the words of Swetaketu, Fou-hi, and Paul then we realize that not all human societies in all historical periods existed in conditions of monogamous marital relationships. For example, Margaret Mead’s Coming of Age in Samoa, first published in 1928, describes the promiscuity of young Samoans (especially females6.) Mead claimed that these young women regularly engaged in premarital sexual relations and that virginity was not prized in this society. In 1983, Derek Freeman claimed that Mead’s descriptions were falsified, claiming that he had interviewed some of Mead’s original subjects who recanted their testimonies given to Mead years earlier. More recent studies suggest that Freeman may have been the one who was fooled by the Samoans, in the sense that the influence of Western values and religion had become greater in the islands since the time of Margaret Mead. What is clear however, is that while virginity is a highly sought after prize amongst the dominant social strata, it has less value amongst the more common people. This means that in Samoan society, bonds between husband and wives are not as strong as they are between brothers and sisters7. This makes sense from an evolutionary perspective, since with high female promiscuity a husband cannot be sure that any child of his wife was fathered by him. Uncles, on the other hand, know that their sister’s child does share their ancestry.
These variations to the rule of monogamy do not change our confidence in its application. In humans, females invest more in offspring than males. Robert Trivers developed theory that predicts that (1) the sex that invests more in offspring will be more discriminating or selective about mating and (2) the sex that invests least will be more competitive for sexual access to the high investing sex. There are two caveats, we are talking about mating or reproduction here, not just sexual activity (although the two are related), the advent of modern birth control technology has allowed freer access to casual and short-term sexual relationships for women and that does not mean that they are choosing the mate they wish to raise their children with. In addition, human reproductive investment is more equitable between males and females than in other mammals and primates.
Sexual selection operates similarly to natural selection, in that first there must be:
1.Variation in traits desirable to the other sex
2. The variation must be heritable
3. Mates must choose the traits, and thereby lead to greater reproductive success in those that carry them.
Women must be able to gauge both present and future potential. Evolution would have favored women who could choose men who possessed those attributes that conferred benefits, such as: physical prowess, athletic skill, industriousness, kindness, empathy, emotional stability, intelligence, social skills, sense of humor, kin network, and social status and disliked men who imposed costs, such as: having children already, bad debts, bad temper, selfish disposition, and promiscuous proclivities. Women must also be able to access these characteristics, particularly if men are able to deceive women. The difficulty of choosing results from the fact that all people are combinations of positive and negative traits and therefore these trade-offs must be integrated into any choice of a mate.
Adaptive Problems in Long-Term Mating and Hypothesized Problems
|Adaptive Problem||Evolved Mate Preference|
|Selecting a mate who is able to invest||Good financial prospectsSocial Status
Size, strength, athletic ability
|Selecting a mate who is willing to invest||Dependability and stabilityLove and commitment cues
Positive interactions with children
|Selecting a mate who is able to physically protectself and children.||Size (height)Strength
|Selecting a mate who will show good parenting skills||DependabilityEmotional Stability
Positive interactions with children
|Selecting a mate who is compatible||Similar valuesSimilar ages
After Buss 19998.
Female Preference for economic resources
Among humans the evolution of women’s preferences for a permanent mate with resources would have three required preconditions, resources must be:
- controllable by males
Secondly, males must have differed in their holdings and willingness to invest these holdings in a woman and her children. These requirements are easily met by humans. Territory, tools, are acquired, defended, monopolized, and controlled by men worldwide. Men differ tremendously in the amount of resources they control, from the homeless to the ultra-wealthy. This has been true historically, even from hunter-gatherer society. Men also differ widely in their willingness to invest their time and resources in long-term relationships.
Women gain more resources for their children from a single spouse (or sequential spouses) than from several temporary sex partners.
Human males invest in their wives and children more than any other primates. In other species females acquire food and shelter since males do not share those with their mates, conversely, human males provide food, shelter, defend territory, and protect children. They also tutor their children in sports, hunting, fighting, hierarchy negotiation, friendship, and social influence. Females must identify cues to signal a man’s possession of these resources. The cues can be indirect, including cues that signal a man’s upward mobility – physical, athleticism, health, social – such as reputation, direct possession of economic resources is the most obvious cue.
In a 1939 study American men and women rated 18 characteristics for a desirable marriage partner. Women rated good financial prospects as important, men rated it as desirable, but not really important. (scale women 1.80, men 0.90). In 1985, in a similar study, women rated it 1.90, and men 1.02. Personal ads in newspapers confirm this. In a study of 1,111 personal ads, female advertisers seek financial resources 11 times more than males. In a cross cultural study of 37 cultures on 6 continents and 5 islands with populations as diverse as coast-dwelling Australians to urban Brazilians, to South African Zulus, some cultures were monogamous (Spain and Canada), others were polygamous (Nigeria and Zambia), and countries were co-habitation is as common as marriage (Sweden and Finland) and places were this is severely frowned upon (Bulgaria and Greece), total sample of 10,047 individuals. The variables included: geography, political systems, ethnicity, religious groups, and all systems of mating. Overall women valued financial resources 100% more than men.
Female Preference for High Social Status
Often evolutionary anthropologists use traditional hunter-gatherer societies as a model of what our ancestral societies might have been like. These suggest that our ancestral societies had clearly defined social dominance hierarchies, with resources flowing freely to those at the top, and trickling down to those at the bottom. There is evidence of the existence of social hierarchy in the linguistic analysis of many groups, the term “head men” or “big men” is found in various South Asian languages, Sanskrit, Hindi, and several Dravidian languages, in North America, we see it in Wappo, Dakota, Miwok, Natick, Choctaw, Kiowa, and Osage; as well as in South America in Cayapa, Chatino, Mazahua, Mixe, and many others.
Therefore, social dominance hierarchy was an ancestral condition for all humans (in other words this behavior had been fixed in Africans before anyone began migrating out of the African continent), thus amelioration of social hierarchy is a derived or recent condition (where this can be said to exist, if it does really exist?) Thus the theory predicts that women tend to desire high social status males because this is a cue to their control of resources. Better social status brings more food, territory, and superior health. Greater social status bestows on children social opportunities missed by children of lower ranking males, for male children worldwide, access to better quality mates typically accompanies families of higher social status.
A study of 186 societies ranging from Mbuti pygmies to Aleut Eskimos showed that higher status men had greater wealth, more wives, and provided better nourishment for their children. Women in the US express a preference for high social status or high status profession, only slightly less important than good financial prospects. In one study of 5,000 college women listed as desirable characteristics of mate: status, prestige, rank, position, standing, station, and high place; considerably more than men. Women at the University of Michigan in 1989 rated 67 characteristics for both short-term casual sex versus a long-term marriage prospects. In this study likelihood of success in a profession and likelihood of promising career were ranked at 2.60 and 2.70 on a 3.0 scale for a potential marriage partner, but only 1.10 and 0.40 in a casual sex partner.
American women place a great deal of emphasis on education and professional degrees – characteristics that are strongly linked to social status. In that same Michigan study women said that lack of education was highly undesirable in a husband, giving it an average rating of –2.39. In other words the cliché that women prefer to marry doctors, lawyers, professors and other professionals corresponds to reality. Women shun men who are easily dominated by other men, or who fail to command the respect of the group.
Male age is also a good indicator of his access to resources. In 37 cultures examined women favored men and average of 3.5 years older. Older men generally have higher access to resources, in contemporary Western societies, male income generally increases with age. American males who are thirty generally make $14,000 more than men who are twenty.
Males in Positions of Power
Marriage patterns in modern America suggest that males with resources can actualize their preferences. High status males can frequently select women a few decades younger (e.g. Rod Steward, Mick Jagger, Warren Beatty, and Jack Nicholson, for example.)
Studies have examined the impact of male occupational status on the physical attractiveness of the women they marry. Men in higher social categories can marry women far more attractive than men in lower categories. In fact, a man’s occupation is the best predictor of the attractiveness of the woman he marries.
This fact is recognized by advertisers who exploit the universal appeal of beautiful, youthful women. Although Madison Avenue is charged with advancing an arbitrary standard of beauty that others must live up to, they are not responsible for the standard, it is evolved and they employ it to their purposes. Indeed this is an example of cultural evolution driven by biological mate preferences; advertisers whose models more closely approach our biological mate preference images will be more successful. So eventually, all existing firms will be doing this (e.g. see Cosmopolitan).
An experiment asked two groups of men to rate the attractiveness of their current romance partners. One group of men was shown pictures of women of average attractiveness and the second group was shown pictures of women who were highly attractive. The men in the 2nd group rated their own romance partners as less attractive than the men shown pictures of women of average attractiveness. They also rated themselves as less committed to their romance partners, as compared to the men who viewed average pictures.
The reasons for these changes come from the unrealistic nature of the images and from the psychology of men. The few women chosen for these ads come from a cast of thousands. In many cases, thousands of pictures are taken for each woman, Playboy takes 6,000 pictures for each monthly magazine and of these a few are selected for publication. So men are seeing, the most attractive women, in the most attractive pose, in the most attractive setting, that has been airbrushed to conform to male desires. Contrast such images to what men would have seen 100,000 years ago. Under these conditions a male might have seen a dozen or so highly attractive women in his life time. Thus the presence of a relative abundance of attractive women today could reasonably induce a man to consider switching mates, but that was highly unlikely when these mating preference mechanisms evolved. The presence of these images is unhealthy for women also, creating a damaging and spiraling competition with other women. In any month’s Glamour magazine, the first pages will be dominated by beauty ads!
Effects of Men’s Preferences on Actual Mating Behavior
A recent experiment examined the responses of men to personal ads placed in two newspapers in the Midwest and the West coast. The mean age of the respondents was 37, with a range of 26 to 58.
Men tended to respond more to women’s ads that women did to men’s.
Men received only 68% the responses of women.
Younger women received more responses than older women.
Mentioning physical attractiveness generated more responses for both sexes, but it produced significantly more responses for women, than it did for men.
Women who wrote ads that conveyed their sexual attractiveness received considerably more responses than women who did not. For men the result was opposite.
These data support the idea that men act on their preferences, specifically when women advertise the qualities of youth and attractiveness.
Actual marriage decisions confirm the preference of men for women who are increasingly younger as they get older. Cross cultural data confirm that these patterns are general. Averaged across all cultures for which data exist, grooms are at least 3 years older than brides. In polygynous cultures the age difference is even greater. Extreme examples are the Tiwi of Northern Australia, where the brides are 2 –3 decades younger.
Effect of Men’s Mate Preferences on Women’s Attraction Tactics
The theory of sexual selection predicts that the preferences of one sex are predicted to influence the forms of competition that occur in the opposite sex. Thus if men’s preferences have exerted an important impact on mating behavior over time, then we would expect to see it in the way women behave, or compete with one another to fulfill what men want. Data that address this question come from the tactics women use to attract men, tactics that women use to derogate others, and self-descriptions of women in ads when they are seeking men. A 1988 study examined the self-reported usage and the perceived effectiveness of 101 tactics of mate attraction. Two samples of participants reported their tactic use, an undergraduate sample and a newlywed sample. Appearance enhancement figured prominently in both samples of women:
I wore facial make-up.
I went on a diet and improved my figure.
I learned how to apply cosmetics.
I kept myself well-groomed.
I used make-up that accentuated my looks.
I got a new and interesting hair style.
In a separate study an independent panel of undergraduates rated these tactics on how effective they would be in successfully attracting a member of the opposite sex. These tactics were rated as 4.71 on a 7.00 scale by women, but only 1.91 by men. A related study examined deception tactics of women, again these all revolved around physical appearance:
I sucked my stomach in when around members of the opposite sex.
I wore a hairpiece.
I wore colored contact lenses.
I dyed my hair.
I wore false fingernails.
I wore dark clothing to appear thinner.
I wore padded clothing.
This study showed that women’s use of these tactics was significantly more effective than male use of such tactics. Women also appear to be sensitive to male mate preferences in their use of derogation tactics toward other women. A study examined the female ratings for 28 derogation tactics. Such comments as:
Made fun of his/her appearance
Told others that he/she was fat and ugly
Made fun of the size and shape of the rival’s body
The rating scale was 1.00 (not at all effective) to 7 (extremely effective.) Derogating a rival’s body was judged more effective when used by females (3.42) as compared to males (2.95.) But an even larger difference appeared when derogating someone’s sexual fidelity. Women said things like:
Called rival a tramp
Told others that she slept around a lot.
Told others that the rival was loose.
Said rival would sleep with anybody.
Calling a rival promiscuous was ranked at 4.44 effectiveness for women, but only 3.45 for men. A second study confirmed these results, ranking derogating physical appearance at 4.15 for women to 3.69 for men, and calling someone promiscuous ranked 4.44 for women and 3.45 for men.
Summary of Mate Preference Theory
Human males have evolved tactics that help them identify a woman’s reproductive potential and future fidelity. Signals of youth and health embody a woman’s reproductive potential: clear skin, full lips, small lower jaw, symmetrical features, white teeth, absence of sores and lesions, and small waist to hip ratio. This preference is not universal in primates, indeed, orangutans, chimpanzees, and Japanese macaque males prefer older females who have demonstrated their ability to reproduce, whereas adolescent females receive fewer matings.
The second large problem is certainty of paternity. Virginity is valued around the world, but it is not a universal cue. More useful are cues to future fidelity. Thus men around the world prefer, attractive, young, sexually loyal wives, who will remain faithful until death. This is not a western cultural phenomenon and no exceptions to these preferences have ever been found. This suggests that male mate preferences are evolved and are deeply engrained. Four sources of data confirm these conclusions:
Men who respond to personal ads show higher response rates to women who claim to be young and attractive.
Men actually marry younger women, worldwide.
Women spend more time enhancing their physical appearance in the context of attracting men than any other tactic.
Women tend to derogate their rivals by attacking their physical appearances in ways paralleling male mate preferences.
Smoke and Mirrors
The evolutionary pressures concerning romantic love are clearly tied to reproduction. This may lead some to question this conclusion with the observation that homosexual is no less strong than heterosexual love. Granted, and this reality was described in an earlier post on this blog (dated September 2009.) However, the capacity for romantic love evolved and it was driven by the benefits of pair-bonding for offspring survival. The signals of this can be seen in the human brain itself. For example, the amygdala (which is considered the most primitive part of the vertebrate brain) plays a key role in processing social signals. The nucleus accumbens and the ventral pallidum receive projections from the amydala and are crucial aspects of the brain dopaminergic reward-learning system. The working hypothesis is that the hormones oxytocin and vasopressin released during romance reinforces social signaling by linking them to the dopamine reward pathways. In a sense we can and often do become addicted to love9.
What we describe as love is thought to be composed of three components (intimacy, passion, and commitment.) This is described as Sternberg’s triangular theory. It has been recently discovered that during the development of intimacy and passion, humans secrete sexual pheromones. Pheromones are airborne hormones that have been shown to play important roles in virtually every non-human animal species. For example, human females show a strong response to male androsterone when they are ovulating10. On the other hand, human males are can sense copulins which are found in female vaginal secretions. When males come in contact with pheromones they increase their sense of a female’s beauty and they are even more attracted to the hormone when the female is ovulating11. During the attraction phase of human mating the brain is stimulated by phenylethlamine (PEA.) This compound acts on the dopamine reward system, but as time passes the brain builds up resistance to this chemical. After the attraction phase (in a successful long term love affair) follows attachment and commitment. Here brain chemistry is influenced by endorphins and oxytocin. Endorphins are hormones that are painkillers but they also produce security, tranquility, and calm. Oxytocins are produced during cuddling and physical intimacy (especially during orgasm.) Finally, after the attachment phase comes commitment. Commitment grows rapidly and then begins to level off (again in successful love relationships.) The production of children (especially ones that the male is confident are his) develops even greater potential for commitment.
Finally, it can be asked if romantic love is so great, than why are marriage rates declining and rates of divorce skyrocketing. My first pass at answering this question comes from “mismatch” theory. Mismatch theory proposes that most modern disease and behavioral pathology is resulting from the differences between the conditions in which are species evolved (small groups, hunter-gatherer, subsistence) and our modern industrial lifestyle (large anonymous society, post-industrial, abundance for some.) In The Race Myth I discussed why there was a disparity in divorce rates between African and European Americans12. One factor that played a significant role (as predicted by evolutionary mate choice theory) was whether an African man was employed or not. Employed African American men were twice as likely to be married as unemployed. Interestingly enough a recent study out of the Duke University Center for Neuroeconomic Studies indicates that there may be a polymorphism in the brains of men that makes them more attracted to either beautiful women or money!13 One can imagine that in the short term being attracted to women might be advantageous but that the men more attracted to money may be those who would be the more suitable long-term female choice. However the significance of such a result is tempered by the fact that in modern society, women are not as dependent upon having a husband to guarantee them material or emotional support for their child. However, there are serious risks to women and children when the woman is in a relationship with a man who is not the father of her children. For pre-school children, the risk of being killed by a stepparent is 40 – 100 times greater than for children with both natural parents14.
There can be no doubt that romantic love is the result of evolutionary adaptation. Without it, it is hard to imagine how our species could have possibly survived. Indeed, some authors conclude that our capacity to love connected with our biological requirement for mating drove the evolution of our excess capacity for thought14. Even today we are preoccupied with it. Ever notice how when you are heartbroken how you cannot seem to avoid a song about love on the radio?? I think Pat Benatar explains all so well:
We are young, heartache to heartache we stand
NO PROMISES, NO DEMANDS
Love Is A Battlefield
We are strong, no one can tell us we’re wrong
Searchin’ our hearts for so long, both of us knowing
Love Is A Battlefield
You’re beggin’ me to go, you’re makin’ me stay
Why do you hurt me so bad?
It would help me to know
Do I stand in your way, or am I the best thing you’ve had?
Believe me, believe me, I can’t tell you why
But I’m trapped by your love, and I’m chained to your side
We are young, heartache to heartache we stand
NO PROMISES, NO DEMANDS
Love Is A Battlefield
Written by: Mike Chapman & Holly Knight
- This is implied in his first letter to the Corinthians, 1st Corinthians, 7:32.
- Brehm et al, Intimate Relationships, (New York, NY: McGraw Hill), 2002.
- Debiec, J., From affiliative behaviors to romantic feelings: A role of nanopeptides, FEBS Letters 581: 2580-2586, 2007.
- Westermarck, E, The History of Human Marriage, (London, UK: MacMillan & Co), 1903.
- 1st Corinthians, 7:1 – 7.
- Mead, M., Coming of Age in Samoa, 1928 (paperback, Harper Perennial), 1971.
- Schoeffel, P, Sexual morality in Samoa and its historical transformations, in A polymath anthropologist: essays in honour of Ann Chowning, (Auckland, AU: Department of Anthropology), 2005.
- Buss, D.M., Evolutionary Psychology: The New Science of the Mind, (Boston, MA: Allyn and Bacon), 1999.
- Debiec, J., 2007.
10. Cutler et al, Human sex attractant hormones: Discovery, research, development, and application, Psychiatric Annals 29(1): 54-59, 1999.
11. Grammar, K, Sex and olfaction, Science 273: 313, 1996.
12. Graves, J.L., The Race Myth: Why We Pretend Race Exists in America, (New York, NY: Dutton), 2005.
13. Smith, D et al, Distinct Value Signals in Anterior and Posterior Ventromedial Prefrontal Cortex, J. Neurosci., Feb 2010; 30: 2490 – 2495 ; doi:10.1523/JNEUROSCI.3319-09.2010.
14. Daly, M and Wilson, M, Homicide, (New York, NY: Aldine de Gruyter), 1988.