1. Because pregnancy requires significant expenditure of energy on the part of the mother, producing offspring is more costly for females than males. Throughout human evolution, females with a strong instinct to stay with their children successfully raised more offspring than mothers who had lots of children but did not give consistent care to any of them. Eventually, the loyalty-to-child gene became a common feature of all women. The pressure to stay with children did not act on males to an equal degree. Because producing new offspring was far less costly for men, they could occasionally benefit from abandoning their children (trusting that the mother would continue to care for them) and going off to start additional families. This evolutionary heritage manifests itself today in the presence of far more single-mother than single-father families. It is not as if parents make the conscious decision to stay or leave depending on what will maximize their evolutionary fitness. Rather, millions of years of past selection leave mothers somewhat more predisposed to stay with and care for their children than fathers. (I wonder if this is a matter of cultural precedent as well as evolutionary trend—we see single-mother families, and so subconsciously we come to expect single-mother families.)1
2. A tendency to accidentally care for someone else’s offspring is not good for Darwinian fitness. Historically, genes that made men complacent about caring for other men’s children were not advantageous from the standpoint of natural selection. These genes were not passed on as often as genes that made men instinctually care only for their own children. While the relationships between parents and children are much more complicated than pure instinct—as I am sure any stepfather could attest—at some basic level, the instinct to care for only one’s own children remains.2 People also want to be with spouses they can trust (another desire ingrained in us by natural selection), and they see adultery as betrayal.
3. Females who are good at identifying males inclined to fidelity are able to raise more children than females who have no preference for faithful males. In some species, males complete specific rituals such as nest-buildings as promises of future loyalty. The traditions surrounding engagement rings are slightly different, because American women do not instinctually expect diamonds specifically. While the details of the tradition are rooted in cultural expectation rather than evolution, the instinct to seek some large indicator of loyalty is still based in our evolutionary past. The size of the man’s investment in the ring indicates that he is seriously committed to marriage.3
4. For the same reason that women are predisposed to appreciate excessively expensive engagement rings, women are predisposed to trust other gestures that suggest commitment. Male seducers take advantage of this tendency, because they know that if they feign commitment, women are more likely to trust them.
5. Individuals attracted to young, healthy members of the opposite sex tend to have more4 offspring than those who are attracted to decrepit, pigeon-cursing vagrants. Since at least part of attraction is due to genetics, the offspring of those attracted to young, healthy partners will also tend to be attracted to young, healthy partners. Eventually this trait came to dominate the population. (Also, awful-smelling vagrants are doubly unattractive, since there is additional selective pressure to be repulsed by awful smells in order to avoid eating inedible objects, contracting diseases from rotting meat, etc.)
1. These gene/culture knots are famously difficult to untangle.
2. The human brain is full of these competing interests like a stepfather being instinctually inclined to care more for his biological children while simultaneously desiring to maintain a reputation in his community as a “fair” person.
3. And the significance of this tradition may lessening as women gradually progress towards being treated equally.
4. (and healthier and who, themselves, are likely to have further offspring)…