The Puzzle of Love.
Is romantic love a universal human emotion? Are we wired to love and stay in love with one person only? These questions have puzzled psychologists, anthropologists, and historians for a long time.
A survey of the anthropological record from 166 cultures, for example, found evidence of romantic love in 88 of them, and concluded that the literature was too thin to make decisive pronouncements on the remaining groups .
There is another common story told by historians. According to this famous Romantic Love Thesis, love was invented by troubadours in the European Middle Ages to promote certain ideals of chivalry, the idealization of women, and the myth of burning, undying love. On this view, this particular myth should be understood as a folk belief found among most westerners (albeit with increasing global appeal) but not a universal human trait .
In his best-selling book on happiness, the cognitive scientist Jonathan Haidt reviews central questions on love and attachments across cultures and adopts a compromise view. Love and romantic attachments, he argues, form an integral part of the human experience past and present, but specific beliefs about the intensity and longevity of love and their desirability have changed over time and vary across cultures. Haidt argues that what the troubadours left us with is a specific set of beliefs about true love that he summarizes as follows:
"True love is passionate love that never fades; if you are in true love, you should marry that person; if love ends, you should leave that person because it was not true love; and if you can find the right person, you will have true love forever" 
For Haidt, the myth of true, everlasting love is not a particularly good one, because it is bound to set us up for disappointments. On this peculiar view, everlasting passion is what seems to be hailed as the desirable goal for two people. Given what we know about human psychology and biology, it is both very unlikely and unsustainable for the kind of obsessive, drug-addiction-like love people initially fall into to last more than a year. Compassionate love, rather, is a distinct kind of longer-lasting bond toward which two people can eventually transition. True love can exist, writes Haidt, but “it is not—cannot be—passion that lasts forever. True love, the love that undergirds strong marriages, is simply strong companionate love, with some added passion, between two people who are firmly committed to each other”. One could propose a 3 Cs recipe for Haidt's recommendation: Compassion, Care, and Commitment.
But if it is not meant to last forever, why do we fall in passionate love, and how do different cultures accommodate this tension between passionate and compassionate love, caring, and commitment?
Love in evolution: a necessary illusion.
The passionate love we initially fall into is like an obsession: it is characterized by the intense channeling of our attention, affection, and desire towards specific people….at the expense of just about everything else! For evolutionary psychologists, romantic obsession operates the convenient trick of duping us into overestimating our prospective mate’s value, construing their features as unique and desirable, and being blind to their flaws. This is a well-documented effect that has been termed the ‘love-is-blind bias’ by psychologists . Overestimation of a romantic mate’s physical attractiveness, personal attributes and overall mate value have been amply documented effects of the this bias. The love-is-blind bias has been likened to other ‘positive illusions’; that is, to unrealistically favourable forms of self-deception, self-enhancement, and optimism biases that help people feel good, avoid discomfort, and ignore negative information in everyday situations .
Father involvement and cooperative child-rearing: specialized evolutionary adaptations.
For scientists of human evolution, romantic love is a necessary illusion: it is a natural bias that evolved as a special adaptation to guarantee our reproductive success in light of a unique challenge faced by our species.
In most of the animal kingdom, younglings are born readily equipped with instinctual dispositions that enable them to fend for themselves very quickly. As humans, however, we remain weak, vulnerable, and slow-learning creatures during very long childhoods, and must rely on others for all our basic needs. What is more, most of the knowledge and skills we come to acquire and require for living in our worlds (think of the importance of language, cooking, and dwelling to name but a few distinctively human institutions)- are learned socially.
To deal with this evolutionary challenge, we have developed a specific neurobiology and highly sophisticated technologies, social institutions and long-lasting forms of cultural learning that enabled us to nurture our slow-maturing large brains and pass on our knowledge for future generations to improve . Along with the importance of culture, pair-bonding, and the continued involvement and commitment of fathers in childrearing has also played a key role in our evolutionary past. And thus romantic love was born! The ‘naturalness’ of romantic attraction and the cultural institutions that celebrate and regulate love, from an evolutionary perspective, enable prospective parents to generate and maintain the sexual and parental commitment that is required for the survival of our species .
Are we naturally monogamous?
Whether lifetime monogamy came as a stable part of these specialized evolutionary adaptations remains an open question. Judging by the evidence at our disposal, humans across cultures seem to attempt, but ultimately fail at being monogamous or remaining pair-bonded with a single person throughout their lifetime . Helen Fisher, one of the world’s leading authorities on the biology of love, has argued that we possess an evolved neurobiological system for pair-bonding that can maintain the intensity of love for an average of four years -- which she cites as an average length for pair-bonded relationships across cultures. This four-year time period, she claims, also coincides with the length of time during which infants are most vulnerable and in need of dual parental involvement . Critics of Fisher have argued that no consistent data are available on average relationship lengths and arrangements throughout history, and that the collective childrearing practices of hunter-gatherers can accommodate a broad range of mating arrangements, with or without the presence of biological fathers .
Monogamy: a specialized cultural adaptation.
In a more recent review of these questions, a team of evolutionary anthropologists led by Joe Henrich showed that long-lasting monogamous marriages were not the norm outside Western countries until very recently . They pointed out that roughly 85% of societies known in the anthropological record allowed men to marry multiple wives. The historical record also shows that lifelong monogamous marriages, as a rare social institution, can be traced back to classical Greece and Rome, and did not begin its global spread until recent centuries .
In their provocative review, they argue that monogamy should be understood as a recent form of cultural adaptation that comes with strong social advantages and unique benefits for women, men, and children.
What’s in it for men?
The authors point out that in polygynous societies (where men marry multiple women) there is more pronounced sexual competition between males, which significantly increases rates of violence. Consider the basic arithmetic of this arrangement: in polygynous societies, men with higher social status will enjoy greater access to women, and higher reproductive success. Low-status men, in turn, will face fierce competition in finding partners. As these unmarried (and socially ineligible) men compete for scarce sexual partners, their testosterone levels tend to raise, which has been correlated with higher crime rates. Although the link between testosterone levels (demonstrated to be lower among married men) and higher crime rates is speculative at this point, the correlation between large pools of unmarried men and criminal behaviour has been amply documented. Similar effects have been found when the male-to-female ratio is unbalanced. For example, a regression analysis examining the effect of the one-child policy in China, showed that a 0.01 increase in sex ratio was associated with a 3 percent increase in violent crimes, even when variables like unemployment and rising inequalities had been controlled for .
As a specialized cultural package that reduces the surplus of risk-oriented unmarried men, monogamy might be an important factor in reducing crime rates.
What’s in it for women?
From an evolutionary perspective, women in polygynous societies appear to have a better deal than men. For one, they are more likely to pass on their genes than low-status men. But high reproductive success of this kind may come with other costs. Because father involvement in childrearing remains low in this cultural package, women typically incur higher parenting costs. They are also likely to marry young, be kept out of education and work opportunities, and compete with co-wives for resources controlled by men. Conflict between co-wives in polygynous systems is another well documented problem. Henrich and colleagues cite a review of 69 polygynous societies from the known ethnographic record, which reports “no case where co-wife relations could be described as harmonious” .
Monogamous systems, in contrast, tend to reduce spousal age gaps, provide more educational and professional opportunities for women, along with more opportunities for gender equity in childrearing and household decision-making.
What’s in it for kids?
For Henrich and his colleagues, stronger paternal involvement is another key benefit of monogamous systems. Higher rates of father investment tend to reduce the burden on women, and also yields higher quality offspring. A number of studies have shown that children from polygynous households have lower nutritional status, poorer health outcomes, and higher mortality rates than their counterpart from monogamous families. The authors also argue that monogamous norms raise the social costs of extra-marital affairs, and shift spousal efforts away from ongoing mate-seeking and toward quality childrearing.
What about polyandry and polyamory?
Polyandry, where women, but not men, are allowed to marry multiple partners has been the subject of fierce controversies in and around anthropology.
A broad consensus still remains that formal polyandry is historically and culturally rare, with a proverbial 28 ancient societies from the Himalayas and Polynesia often cited as exceptions to the rule. Proponents of the "polyandry was more common than you think" thesis like to cite a recent review by anthropologist Karen Starkweather, who claims to have found evidence of polyandry in 50 societies. Starkweather points out, however, that the incidence of polyandrous marriages in these societies only ranged from 9 to 50%. 
A further controversy exists on the extent to which the alleged sexual permissiveness found in many Amerindian societies in Lowlands South America counts as polyandry or polyamory (where both men and women can have multiple partners). The Huaorani of Ecuador and Mundurukus of Brazil, like many Amazonian societies, permit married women to have non-procreative sex with most of their husband’s brothers and parallel cousins, albeit with some measures of discretion .
As an anthropologist, I would caution against applying our own classifications of human societies on people who would not define themselves in those terms. It is important to note that the Huaorani of Ecuador, who may find our puritanism about extra-marital affairs quite strange, would not describe their own societies as "polyandrous" or “polyamorous”, or indeed as ones in which cuddling with an affine counts as an “affair”.
What about modern polyamorous communities?
Polyamory as it is practiced among Moderns has also been the subject of countless blogs, books, and calls for a new sexual revolution. While it may prove to be a rewarding practice for young people as they search for meaning and a place in the world, no convincing longitudinal study so far has given us any indication that children raised with this specific kind of diffused parental involvement in modern environments are better off than their monogamously raised counterpart.
It may very well be, then, that the long-lasting forms of compassionate, committed pair-bonding found in monogamous systems is the best cultural adaptation devised so far to deal with our evolutionary uniqueness. If we combine it with the collective childrearing practices found in small-scale societies and the extended family investment honoured in more traditional cultures, we may have found a recipe worth preserving.
***Update*** - a note of caution to readers on the importance of context and asking good questions:
Something to bear in mind when reading about statistical generalizations such as “no case where co-wife relations could be described as harmonious [in 69 polygamous societies]” is that information in such simplified format tells us very little about the different contexts being studied, by whom, and with what assumptions.
Bambi Schieffelin, a distinguished Professor of Linguistic Anthropology at NYU and the author of several books on socialization and childrearing across cultures champions an approach to the study of people’s lives in their context that emphasizes long-term follow up. We cannot make sense of how “harmonious” relationships might be anywhere, she explains, “unless someone really followed marriages over time, and defined the parameters and contexts, and could figure out what was serious conflict and what was not” (personal communication”).
Schieffelin studied the socialization of children among the Kaluli of Papua New Guinea, a society where men can marry up to three wives if they have the resources or lineage connections. For the Kaluli, she reports that “sometimes the relationships between co-wives were harmonious - and some times they were less so. Just like marital partners everywhere” (pers. comm.)
As far as the treatment of children was concerned, she found no evidence of less-than equal treatment among children of different co-wives. In her view, the assistance provided by older sisters and a broad network of affines [relatives through marriage] in childcare amounted to a very supportive arrangement for all involved.
These insights should serve as a reminder that we must be very cautious about making generalizations on societies based on their marital systems or modes of social organization, and thereby assume that they will be the same everywhere.
The parameters behind research questions are also very important. One could certainly imagine a large-scale survey of monogamous marital units in the West finding “no evidence of harmonious relationships”, and subsequently concluding that monogamy “does not work”.
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