Sarah Blaffer Hrdy has produced two books that provide insight into the ways in which human males and females have evolved and developed reproductive strategies to insure that their genes are passed on: Mothers and Others: The Evolutionary Origins of Mutual Understanding, and Mother Nature: Maternal Instincts and How They Shape the Human Species.
Compared to other mammals, primate males have a significant role in raising offspring. They play a significant role in providing protection and nourishment for infants. In most species, the females are the foragers, and the males are the hunters tasked to provide the valuable protein-laden meat. The degree to which males are diligent in performing these roles is species-specific, and usually limited to infants thought to be their own offspring. Infanticide is common among some species. While a female is suckling an infant she is not available for mating, and it could be several years before weaning will occur. Males, particularly unfamiliar males, often become impatient and kill the infant in order to render the female again available.
Human males are distinguished by their lack of predictable nurturing traits.
The differences in male response seem to be associated with differing reproduction strategies assumed by females. Females realize that they and their infants are better off if the assistance of a male is available. So there are two options available: to mate with a single male and hope that it will be sufficiently reliable in performing its duties, or mate with a number of males and let them all assume they might be fathers and perhaps collectively they will provide enough security—and at least not kill the infant.
While a mother can generally be sure that an infant is actually hers, a male has no guarantee of paternity. He also has two options: trust that the mother is nurturing his infant, or increase his chances of siring an infant by inseminating as many females as possible.
Different species have adopted different strategies, but in most primates females mate with multiple males over a period of time. Hrdy seems to hint that females must have been driven by male irresponsibility to develop such behaviors. It would seem equally likely that males might have been driven by concerns about female promiscuity.
In any event, there are evolutionary consequences and certain traits will be selected and enhanced. For example:
Primate testes size seems consistent with these mating modes, with humans being intermediate in scale.
Following this line of logic, it would seem that human males might have encountered an uncertain reproductive environment where there was variation in mating habits either over time or over space as different bands of humans utilized differing strategies. There is evidence from encounters with primitive hunter-gatherer societies that mating and nurturing arrangements were highly flexible.
More consistent nurturing responses related to mates and infants were presumably never sufficiently selected, leaving men with a more random approach based on their individual attributes. For example, men undergo some of the same hormonal and chemical changes women do, including a lowering of testosterone when in the presence of an infant, but the scale of these changes is much lower in men and highly variable.
The understanding of long existing evolutionary imperatives can shed light on the bad behaviors that seem endemic within men (and women) in our societal environment. Fortunately, evolution has also provided us with a consciousness that allows us to decide to override the more primitive instincts—some of the time.
Hrdy provides us with a related bit of data that addresses a fundamental question that has long puzzled humans. The human male has evolved a rather large penis as primates go. It would seem that thousands of generations of women have voted and the results are conclusive—size matters!