KeywordsMate Choice Marital Satisfaction Marriage Rate Pair Bonding Stable Marriage
Marriage is a sociosexual and socially recognized bond of some duration between a man and a woman. This is the traditional definition of marriage, although other sociosexual partnerships may meet the criterion.
An evolutionary model of marriage begins with recognition of the centrality of reproduction to the relationship. The economic benefits of marriage, such as economies of scale and specialization of labor, are essentially subordinate to reproduction; raising children is costly in terms of wealth and labor.
Marital behavior that maximizes reproductive success will be favored by natural selection. Marital behavior seems to have evolved in order to keep the parents together and available to raise their common offspring. Infants and children are highly dependent on their parents and develop better when raised by their biological parents than in most if not all other arrangements. This is true not only of infants but also of children and adolescents and applies to health, psychological functioning, cognitive performance, antisocial behavior, and survival itself (Geary 2005; Cabrera et al. 2000).
The functional hypothesis that marriage evolved mainly in order to promote biparentalism is supported by the fact that pair bonding, the rough animal equivalent, tends to occur in mammals with highly dependent young. In essence, in these species the male tends to remain with his mate rather than pursue other sexual opportunities. However, even in the primates, in which the young are highly dependent on maternal care, pair bonding is the exception rather than the rule. Another factor recognized in the evolution of marriage is the opportunity for effective mate guarding.
Marriage is universal, and almost all adults seek to marry, and most do marry, sometimes more than once. Marriage confers health and emotional benefits on both sexes (Field 2011). People who do not marry tend to fail to attract a partner, such as impoverished men. These facts suggest that adult humans possess what might be called a pair-bonding motive, perhaps best termed amorousness. In a US survey, almost no one not in a romantic relationship renounced seeking such a relationship for any appreciable length of time. Amorousness is distinct from the sex drive, although from puberty on, a sexual component seems invariably to accompany amorous feelings toward the partner. The capacity for amorousness is universal (Jankowiak and Fischer 1992). Even in arranged marriages, amorousness tends to develop afterward, although less often than in self-selected marriages. The evolved basis of amorousness is further supported by the fact that particular brain areas are activated by viewing a photo of the loved one, including the anterior cingulate cortex (Fisher 2006). Brain areas involved in pair bonding tend to have vasopressin or oxytocin receptors; these hormones play a role in pair bonding and parental care in rodents (Bartels and Zeki 2004). Orgasm causes a surge in oxytocin in both sexes that enhances the bond between them. Relationship satisfaction in women is correlated with oxytocin levels and in men with vasopressin levels (Taylor et al. 2010).
The adult sex ratio influences the marriage rate. When men are in short supply, a lower percent of women marry since men can secure sexual opportunities, sometimes with multiple women, without having to marry. When women are scarce, more are able to secure the advantages of marriage for economic security, companionship, and childcare assistance. Under these latter conditions, their marriage rate rises, and the divorce rate falls.
It has been observed that the contemporary African American community suffers from a dearth of marriageable men, with the result that the great majority of children are raised by a single mother, often assisted by her mother. This matriarchal family arrangement is not a carryover from slavery since around World War II when good jobs for black men were abundant, 80 % of adults were married (Harris 1974). Only with the return of white veterans and the subsequent loss of factory jobs for black men did the marriage rate begin its decline. Economic factors more than cultural factors drive marital dynamics.
The marriage rate is low where men have difficulty finding work that allows them to help support a family. A job that allows a man to support only himself generally renders him unmarriageable and raises the likelihood of divorce. The marriage rate in a society tends to be correlated with economic opportunities, particularly for men. Where good jobs for men are scarce, fewer men marry and the age of marriage rises, as men need more time to accumulate sufficient resources and skills to afford marriage. In modern societies with companionate marriage, women’s age at marriage tends to track men’s, so with the current decline in economic opportunities for young people, men and women are marrying later than in the recent past.
In modern societies with comparatively late marriage, women tend not to be protected against premarital pregnancy by the period of adolescent subfertility. Hence an adolescent girl can get pregnant and become an unwed young mother, the consequences of which are usually grim for both her and the child. The rate of premarital pregnancy and hence single motherhood is higher in countries such as the USA where confidential, free contraceptive services are not provided in secondary schools.
The marriage rate has been falling in Western societies since the Industrial Revolution. Fewer women are marrying, and more women are divorcing. The main reason seems to be that women’s economic opportunities have been rising steadily: salaried jobs are open to them, and they are no longer bound to the family farm (Goode 1993). Many of them can support themselves, and even raise children independently. Cultures in which women and men labor independently tend to have higher divorce rates.
In most tribal cultures, a man must furnish a bride price or labor in order to procure her from her father. The fact that a man often needs many years in order to acquire the requisite skills or resources helps to explain the fact that husbands tend to be older than their wives across cultures. The age disparity is also explained by the breeding imperative: a woman who is just reaching the onset of fertility is at her maximal reproductive value for a prospective husband. Accordingly, women in tribal societies tend to marry, or be married off, about 2 years after menarche, when the period of adolescent subfertility has just passed. This also ensures that she is unlikely to be pregnant.
The criteria of mate choice show considerable consistency across cultures. Both sexes seek signs of health and genetic quality in a prospective spouse. Physical attractiveness tends to be enhanced not just by signs of health and vigor but also by indications of full reproductive maturation. Masculine men and feminine women tend to be more fertile than less sex-differentiated individuals. Women also seek men who are high in social status, act with self-confidence, exhibit nonverbal dominance displays, and secure valued resources (reviewed by Weisfeld 1999). Ethologists define dominance as success in fights or in gaining contested resources, and female animals generally seek dominant males. It seems that women also seek a dominant mate.
Spouses tend to be moderately similar genetically. This seems to be achieved by a combination of olfactory and visual cues. The sexes are repelled by olfactory cues that indicate close genetic relatedness (Jacob et al. 2002; Weisfeld et al. 2003). This guards against inbreeding depression effects and enhances the immunocompetence of offspring. Less well known is the phenomenon of sexual imprinting. In many birds and mammals, the offspring imprints on the visual image of the opposite-sex parent and uses this image in seeking a mate at maturity. This mechanism guides the offspring to pursue mates of the right species, gender, and state of maturity. Men and women tend to marry someone who resembles the opposite-sex parent, thus enhancing genetic similarity of spouses. Preference for mates of moderate genetic similarity has been demonstrated in insects, birds, and primates, so it must be generally adaptive. One explanation is that moderate homogamy, or positive assortative mating, preserves combinations of genes that are adaptive in the parents’ (and offspring’s) environment. Moderate homogamy enhances fertility in humans as well as other species (Rushton 1988).
Another important criterion in mate choice is the apparent commitment of the potential spouse. The resources that a man commands will be of no value to the woman unless they are directed toward her and her children. Likewise, a man may not only squander his resources if his wife is unfaithful; he may wind up aiding a rival. A man may employ a range of tactics to ensure a woman that he is passionately committed to her, including offering gifts, pledging his devotion, and complimenting her extravagantly. A woman may test a man’s commitment by deferring sexual involvement, provoking his jealousy, and raising the question of marriage. She can ensure him of her commitment by the unambiguous expedient of having sex with him exclusively. Both sexes may employ various mate guarding tactics to detect cheating by the other, including, nowadays, constant monitoring by cell phone. The tactics of courtship and marriage may be categorized as self-promotion and disparagement of rivals, evaluation of the potential or actual mate, and retention of the mate.
Marital Satisfaction and Stability
One would expect that these universal criteria of mate choice would, if met, lead to happier and more stable marriages. That is, selection would have favored criteria that led to stable marriages and resulting enhanced fecundity. This seems generally to be the case. Young brides go on to produce more children than older ones. Wealthier men raise more children successfully than do poorer ones. Wealthier men are less likely to divorce and more likely to be polygynous. Couples who are similar tend to stay together.
Analyzing data on scores of cultures, Betzig (1989) identified common reasons cited for a divorce. These included sexual refusal, suspected infertility, husband’s cruelty, and wife’s infidelity. Other research has pinpointed having a handicapped child, death of a child, sexual dissatisfaction, and rape of the wife. All these factors in divorce make sense in terms of reproductive failure. Furthermore, around the world the more children a couple have, the lower the likelihood of divorce (Goode 1993). Divorce is more likely in marriages without children, and peaks at about 4 years of marriage. The first few years of marriage seem to provide a trial period for deciding to have children or not and for testing fertility with the spouse. After 2 or 3 years of a romantic relationship, the period of intense infatuation has passed, freeing the couple to shift their affection to the newborn. In a sense, the spouses are now kept together by their baby.
Another cross-cultural criterion of mate choice is kindness (Buss 1989). Recent cross-cultural research has shown that lack of kindness is an important factor in marital conflict, along with sexual problems, issues regarding child rearing, disputes about division of labor, and economic difficulties (Dillon et al. 2015). Kindness would seem to be essential for an enduring marital relationship. Similarly, in the same five cultures, it was found that the spouse making one laugh was correlated with marital satisfaction of both husband and wife, but especially the wife (Weisfeld et al. 2011). Many women seek a mate with a sense of humor, so this criterion seems to be valid in terms of subsequent marital satisfaction. Having a funny husband did not seem to enhance the wife’s marital satisfaction because it was a sign of intelligence, however. Mediation analysis revealed that having a funny spouse enhanced the spouse’s satisfaction even more by being a sign of kindness, understanding, and dependability.
Developmentally, marital satisfaction tends to decline over time. This does not seem to be related to any adverse emotional effects of having children, at least in collectivist cultures with a thriving extended family (Dillon and Beechler 2010). The probability of divorce declines over time as well. This is one indication that marital satisfaction and marital stability are somewhat independent. This is also the case in cultures that forbid divorce; unhappy couples are forced to remain together.
Sex in Marriage
Sexual satisfaction and sexual frequency are highly correlated with marital satisfaction. In tribal cultures sex is reported to occur nightly except during periods when it is taboo. Sex is usually taboo during menstruation, often taboo during pregnancy, and seldom taboo during lactation. In the absence of contraceptive use, conception typically occurs within 1 year of the onset of coitus. Inter-birth intervals for our species are about 3 years, so women are perennially pregnant or lactating. Breastfeeding typically lasts for about 3 years, with solid food introduced after about 2 years. In simians nursing usually stops when the next pregnancy begins, if it has not already ceased.
Sexual infidelity is associated with low marital and sexual satisfaction, permissive values, low economic status, and opportunities. Men report more infidelity than women, and attractive men with wealth and high testosterone report more infidelity, at least partly because of greater opportunities. In US research, women with low self-esteem reported more infidelity. In a cross-cultural study, sexual infidelity for both sexes was correlated with low love of the spouse, finding others sexually attractive, and the spouse being unfaithful (Nowak et al. 2014).
Like other valued relationships, pair bonds tend to be defended against interlopers. For example, children tend to show jealousy if their parent favors their sibling (Dillon 2013). Rivalry for parental resources tends to be greater between half-sibling and stepsiblings than full siblings.
Lovers too exhibit sexual and romantic jealousy; mate guarding is typical of pair-bonding species. If in human evolution there had not been at least occasional instances of infidelity, jealousy would not have evolved. Estimates of the frequency of marital infidelity vary but are not insubstantial. Infidelity may raise a husband’s reproductive success if he succeeds in fertilizing another woman. Infidelity may benefit a wife if she cuckolds her husband and thus deceives him into caring for a child she has had by a man of greater genetic quality than his or garners resources from her lover. Jealousy in men tends to be evoked by the threat of the wife’s sexual infidelity more than by the threat of her desertion; the opposite pattern is more typical of women. Suspected infidelity is the principal cause of homicide around the world.
In a cross-cultural study, both sexes worried about spousal infidelity more if the spouse was attractive (Dillon et al. 2014). Worry was greater when the spouse socialized independently. Individuals who felt possessive about the spouse touched the spouse more. Worry was greater in persons who were themselves unfaithful.
Since marriage is essentially about reproduction, it seems appropriate to consider child-rearing behavior within marriage. Longitudinal research reveals that when a man marries or becomes a father, his testosterone level falls, and if he divorces, it rises (Mazur and Michalek 1998). One effect of testosterone is to increase competitive behavior; a married man has less need to compete for mates than before. Similarly, husbands with comparatively low testosterone tend to be more family oriented, whereas those with higher testosterone are more likely to divorce and to earn less money. When a woman becomes pregnant, she and her spouse experience a steady rise in prolactin, one of the several parental hormones (Storey et al. 2000). The higher his prolactin level after the birth, the more nurturant he is, as indicated by emotional responsiveness to a crying baby. Prolactin also suppresses testosterone, resulting in further nurturance.
Pregnant women undergo a rise in at least nine hormones. Several of these, including oxytocin, estrogens, progesterone, and prolactin, enhance maternal bonding and behavior. Prenatal and pubertal hormones contribute to the sex difference in nurturant behavior observed throughout the life span in humans and other primates. Breastfeeding maintains the levels of the lactational hormones prolactin and oxytocin.
Men and women are hormonally disposed to care for their children, but the mechanisms are quite different. Consistent with this, women seem to be closely bound to their infants, whereas men are relatively more attracted to weanlings. When a second child is born, the father is inclined to assume some of the care of the weanling, while the mother attends to the newborn. Some Scandinavian countries provide fathers with their own period of paid paternal care that they can take after the first year or so of parental leave expires. Fathers often take their sons on excursions during this time. Around the world, men specialize in tutoring their sons in subsistence skills, protecting the family, performing tasks with hard and heavy materials, serving as warriors, and providing more resources given that wives devote more time to child rearing and domestic tasks (Mackey 1996). Mothers are the main teachers of girls’ subsistence and child-rearing skills.
In every culture, women provide more childcare than men on average. The parents are typically assisted by grandparents, older siblings especially daughters, other kin, and other women. Some cultures are described not as having the extended family but as having the nuclear family. However, this invariably means that the grandparents do not share the parents’ dwelling but do reside nearby in a family compound. Thus the norm for our species is care by the extended family. Bonds of kinship are established and maintained by olfactory cues, kinship terms, social contact, social pressure, and other factors. In addition, adolescents undergo a period of intense tutoring by a same-sex adult. This instruction pertains to social and tribal obligations and often culminates in a puberty rites ceremony. Children also learn from imitating and being instructed by older children in these cultures characterized by little age segregation.
Divorce typically reduces contact time with the father markedly and imposes financial loss due to the need for separate residences and other appurtenances for the ex-spouses. Children of divorce are essentially raised by a single parent in terms of average developmental outcomes. If the mother remarries, the consequences for her children are often no better than if she remains unattached. Stepchildren are at risk for neglect, abuse including sexual abuse, and even homicide and accidental death. In a US study, only about half of stepfathers and one-fourth of stepmothers reported any parental feelings toward their stepchildren (Duberman 1975).
What has been presented thus far is an overview of human marriage – the normative pattern for our species. But human behavior is very flexible, and marriage takes somewhat different forms around the world. In most modern societies and in tribal societies where little property is available for bride price, marriages are usually not arranged by parents – although parents seem to exert considerable influence over children’s choices. In these cultures a formal marriage ceremony is especially common, perhaps as a way of displaying wealth, discouraging divorce after a public commitment, and arranging economic alliances with in-laws. Marriage ceremonies occur in the great majority of cultures (Quale 1988). In all cultures the double standard of less acceptance of female promiscuity than male promiscuity is in evidence to some degree. In some cultures sex roles are strictly prescribed, whereas in modern societies these are being mitigated. However, even in modern societies, men gravitate toward jobs involving hard manual labor and women toward service occupations. Cultures vary in the status ascribed to women compared with men. In most cultures, especially bellicose ones, women are expected to defer to men in various ways, and sons are privileged over daughters. Other societies are more egalitarian, and women live much longer than men, unlike “male supremacy” cultures, where the gap is smaller. In most cultures a man must qualify for marriage economically. Among wealthy people in highly stratified societies, a woman’s family may offer her a dowry to enhance her marriage prospects for marrying into another wealthy family. She will also be pressured to remain a virgin until marriage, as a way of raising her mate value and of discouraging her from eloping with a penurious cad. For an analysis of marriage systems, in particular subsistence conditions, see Quale (1988).
One particularly interesting variant is polygyny. This was permitted in the majority of tribal societies, although only a handful of men could afford more than one wife. Highly stratified societies, such as the USA, effectively have serial polygyny when wealthy men divorce and remarry to found another family. Polygyny increases the reproductive fitness of the husband, but at the expense of men left unmarried. Women with co-wives have almost the same fecundity as monogamous wives, and certainly have more children than unmarried women. The availability of the polygyny option seems to benefit women; they may choose to marry a wealthy man who is already married, rather than marrying a destitute man or remaining unmarried.
Polygyny is favored in economically stratified tribal societies. It becomes more common when the sex ratio falls, as after a war: a defeat brings a shortage of able-bodied men; a victory offers women to be captured. Polygyny is also favored by poor economic opportunities for women, who must marry for a livelihood.
Polygyny has some advantages, cited by its defenders. It raises the marriage rate for women and the percentage of children brought up by their biological parents. Co-wives can cooperate in various ways, such as childcare and conspiring against the husband. Co-wives are often sisters; if so, child survival is higher. A young wife may be favored sexually by the husband, but first wives tend to have more power.
The field of marriage research has been largely atheoretical. Factual data abound, but conceptual integration of these data has been lacking. Many studies deal only with a single society, particularly the USA. Some standard facts are trivial, such as that spouses tend to have lived near each other before marriage and that spouses tend to have similar values. Until fairly recently, most research on marriage surveyed only the wife. None of the commonly used measures of marital satisfaction has been validated by invariance testing for cross-cultural use. Only the Marriage and Relationship Questionnaire (MARQ) developed in England by Russell and Wells (1993) seems to have had scales validated across cultures and genders (Lucas et al. 2008).
The emerging evolutionary approach to the topic of marriage has offered some distinct advantages. The fraught topic of sex differences in married people has been addressed seriously for the first time. Previously, husbands and wives have been treated largely as interchangeable parts. Similarly, much of the social science literature neglects the factor of kinship. Huge effects of stepparenthood have been disclosed by evolutionary scholars. Unfortunately, the US Census Bureau does not inquire about biological parenthood in its surveys of households.
Evolutionary psychologists have been somewhat slow off the mark in studying marriage. In a way, married couples have been victims of the success of the field in studying mate choice. Hundreds of studies have been conducted on mate choice, many of them on US college students in dating relationships. Conducting research on married adults has been more methodologically challenging. Particularly rare has been ethological research on married couples, along the lines of Weisfeld and Stack (2002), who found that wives gazed longer at the spouse than husbands, whose glances were brief, and wives laughed and smiled more. Also needed is more cross-cultural research, especially on diverse cultures. If, say, a sex difference is found in a variety of cultures, the likelihood of an evolved basis is higher than if it is found only in Western societies. A volume by Weisfeld et al. (in press) is intended to address some of these shortcomings.
Another area in need of further work is the emotional basis of various marital behaviors. Mate retention tactics, employed outside and inside marriage, might be analyzed with regard to the emotions involved in execution and the effects on the partner. For example, several emotions seem to be involved in the threat of mate loss; they might be analyzed in detail. The emotion of amorousness is seldom mentioned as being a basic human emotion, yet it seems to qualify as one. Sexual behavior and emotions seem to vary with the likelihood of sperm competition, another topic that has emerged recently.
Also making great contributions to our understanding of marriage is research on the brain and hormones. These findings have the felicitous side effect of helping to dispel the notion, prominent in the social sciences, that social behavior is simply the product of experience and culture. Demonstrating that a particular neural mechanism mediates some marital behavior or capacity is one sort of information confirming the evolved basis of that behavior. Other methods include showing the specific effect of a hormone and establishing that the behavior is universal. Demonstrating that a given behavior has an evolved basis rather than being a product of our domain-general capacities is necessary for delineating human nature. Once a behavior can be shown to have an evolved basis, questions of its function can be addressed.
The great promise of an evolutionary analysis of marriage is that comparative analysis, including analysis of pair bonding in various species, will lead to elucidation of the function of particular marital behaviors. Function is a unifying concept that can bridge disciplinary boundaries and make common sense of otherwise baffling research findings.
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