Sperm competition is a factor in some cases of paternal uncertainty, but there are also many cases in which it is not. For example, Daniel Boone’s wife had a daughter, Jemima, when he had been gone for more than a year. There is reason to believe that Jemima was fathered by Daniel’s brother Edward. Non-paternity, but no sperm competition. Adoption also creates non-paternity without any sperm competition. Generally speaking, sperm competition ought to be more likely in a species that has a seasonal estrus cycle – all effective mating attempts have to be close together in time.
In a case where there is sperm competition, we can assume that the non-official parent fertilizes half the time. This means that 2 times the frequency of non-paternity is an upper limit on the fraction of conceptions in which sperm competition occurs.
Since every recent, high-quality set of genetic measurements shows that non-paternity is currently low and has been low for hundreds of years, usually under 2%, for those populations that have been studied, the fraction of conceptions in which sperm competition occurs is less than 4% (in those places and times). We have an upper limit – but is it a good upper limit? Is it close to the actual rate?
Once in a while, women give birth to fraternal twins that have different fathers. The frequency of this event can give us a direct measure of the fraction of conceptions in which sperm competition actually occurs. In the presence of sperm competition, we only notice the cases in which the twins are discrepant. The incidence of sperm competition will be twice that fraction.
So what is the rate? This is the sort of question that could be answered definitively nowadays with a SNP chip, but here’s what we have: In an Italian study from 1992, the frequency was 2.4% among fraternal twins whose parents were involved in paternity suits – people who presumably had reason to be suspicious. Generally speaking, nonpaternity is much more common in high-suspicion cases of this sort, usually at least 10 times more common than in the general population. Another 1992 estimate was that 1 in 400 pairs of twins born to married white women had different fathers, which give a sperm competition rate of about half a percent – reasonably compatible with the Italian numbers.
Maybe we need to redo this with modern genetic techniques. Maybe Italian girls are special. Maybe everything was different back before recorded history.
But if these numbers are correct and at all representative, sperm competition in humans is insignificant, and all the people talking about our specialized somatic and behavioral adaptations for sperm competition are wrong.
Someone mentioned that I am calling people loons who’ve authored of dozens of peer-reviewed papers and are at serious (sic) universities.
I can only say – somebody’s gotta do it!