In humans, females have greater reproductive certainty than males. This means that while most females will eventually reproduce, a lot of men are can be entirely excluded from reproducing.
Also, since human females can produce a limited number of eggs and invest much more in their offspring than males, they’re a reproductively valuable resource.
The result is that there’s usually fierce
among men for women and men are predisposed to be aggressive, eager to mate, and less discriminating in choosing mates
Now, men of higher mate value who have resources and are attractive can achieve reproduction by means of attraction with willing women but what about men of lower mate value?
Men of low mate value who lack resources and are unattractive have extra psychological pressure to achieve reproduction whenever they can and so may resort to sexual aggression and rape in a desperate attempt to pass on their genes.
This is why a huge proportion of rapists tend to be poor and ugly, having a low facial symmetry which indicates poor genetic quality.
This, however, does not mean that only sexually deprived men of low mate value commit rape. In their quest for greater reproductive success, men who achieve mating through the means of attraction can also resort to sexually aggressive tactics in circumstances where costs are outweighed by the benefits.
Take war for example. Rape is common during wars because not only do the aggressing men eliminate other men who would otherwise guard their women, there’s no law and order that can hold them accountable for their actions during such times of chaos.
Another circumstance in which men, not necessarily of low mate value, can rape is when they come to know about or suspect their partner’s infidelity. Partner rapes, especially during a breakup, comprise a significant proportion of reported rapes.
Concerned by the possibility that some other man may have inseminated her partner, the man uses force to inseminate her so that he can win the sperm competition by beating the other man’s sperm to the egg.
Rape in nature
Humans are not the only animals in which sexual aggression and rape are observed. Although rape is a rarity in the animal kingdom, the males of species as diverse as insects, ducks and monkeys have been observed engaging in sexual coercion to force insemination.
In a male scorpionfly, for example, there’s an organ specifically designed to facilitate sexual access to a female in a coercive manner. It’s a type of hook on its wing that enables it to grip the female as it forces copulation.
Though all males in this species have this organ, they don’t all use it. Females of this species prefer to mate with males who bring them a nuptial gift (a dead insect as food). When a male fails to offer food, females aren’t interested in mating and that’s when the coercive technique is employed by the male.
|A male scorpionfly with its clamper.
Similarly, small orangutan males who’re unable to achieve copulation through intrasexual competition with other bigger males (small size is not good for intrasexual competition) chase and rape the females.
The white-throated bee-eater is a species of bird in which rapists are those individuals who, after breeding early in the season and raising young with a monogamous partner, embark on raping forays.
They chase any female who is still fertile and who has been left unguarded by a male and attempt to force insemination. Clearly, the males of this species are going for greater reproductive success.
McKibbin, W. F., Shackelford, T. K., Goetz, A. T., & Starratt, V. G. (2008). Why do men rape? An evolutionary psychological perspective. Review of General Psychology, 12(1), 86.
Stanford, C. (2009). Despicable, Yes, but Not Inexplicable.