Everyone knows that men can never really know that their children are biologically their own, short of a DNA test. But most people think that, well, cases of mistaken paternity are rare freak cases, and the people we know couldn't possibly be cases of mistaken paternity; and most men figure that when they have kids, of course the woman they choose will be trustworthy and faithful, so it's a problem for freak-show losers to worry about.
While reading Gigerenzer's Law of Indispensable Ignorance on Edge.org's Edge Annual Question (this year, "What's Your Law?"), I came across the startling sentence:
The estimated 5 to 10% of children and their fathers who falsely believe that they are related might not lead a happier life by becoming less ignorant; knowledge can destroy families.
"Holy crap! 5-10%? That's got to be a mistake," I thought. Is it? Like any lazy researcher, I did a quick Google search, which turns up a thread from the sci.anthropology newsgroup archives with the rather insensitive, albeit humorous, subject "How many bastards are there, anyway?". At first, it seems, we can breathe a sigh of relief, as Lee Rudolph writes:
Yesterday I was talking to a bioethicist who studies genetic counseling, and when the conversation turned to folklore, she brought up an issue she'd recently investigated. She told me that essentially all genetic counselors believe that there is a "false paternity" rate of about 5 percent--yet few if any counselors encounter a rate nearly that high in their own practices. This discrepancy having piqued her curiosity, my friend had tried to track down just why the rate is believed to be 5 percent. One source after another said the scientific equivalent of "it was published by a friend of a friend". Eventually the FOAF-chain terminated, in a single paper whose author does not think it supports the interpretation it is popularly given! Appendix 1, immediately below, represents the present state of her research into this possible piece of genetic folklore.
... [skipping to appendix] ...
A historical search and a conversation with Dr. James Neel, one of the deans of human genetics, led us to what may be the source for the high estimate: a 1962-65 study of blood typing in a small Michigan town (1). That study found discrepancies between biological and stated parentage in 109 of 2507 nuclear familes. Many of these may have been unacknowledged adoptions, including step-parent adoptions.
...[skipping to cite]...
(1) Sing, CF et al (1971) Studies on genetic selection in a completely ascertained Caucasian population II. Family analysis of 11 blood group systems. American Journal of Human Genetics 23(2) 164-198.
Whew! OK, so it's one small study, and it may not have counted adoptions properly. But hold on --- a few messages later in the thread, Joe Quellen writes:
The statistics are covered in a very engrossing book by Jared DIamond, called *The Third Chimpanzee*, 1992, Harper Perennial, Chapter 4, The Science of Adultery. He describes a study of blood typing and genetics which had unexected results and was quashed. It was done in the 1940s at a "highly respectable" US hospital. The study found that fully 10 percent of babies were not the biological offspring of their legal fathers.
Oh. Diamond was citing a different study. A couple of messages later, "sgf" writes:
I just picked up Timothy Taylor's _The Prehistory of Sex: Four Million Years of Human Sexual Culture_ (Bantam: 1996). Here's his summarization of the subject:
"...In tests of genetic paternity recently conducted by Robin Baker and Mark Bellis , they found that around 10 percent of children had been sired by someone other than their ostensible fathers -- although the fathers consciously believed these children to be their own.
...[skipping to cite]...
[1 Baker, R. and M. Bellis. 1993 "Human sperm competition: ejaculate adjustment by males and the function of masturbation." _Animal Behaviour_ 46: 861-65]
And just to pile on a bit, the next Google hit is a working paper by anthropologist Kermyt G. Anderson titled "How well does paternity confidence match actual paternity? Evidence from worldwide nonpaternity rates" (abstract; DOI) (UPDATE 2010-02-17: fixed linkrot), wherein we learn:
The median nonpaternity rate for the high paternity confidence sample is 1.9% (range: 0.4 - 11.8), while median nonpaternity for the low paternity confidence sample is 30.2% (range: 14.3 - 55.6). The median nonpaternity rates for these two groups are significantly different (Wilcoxon sign-rank test, z = -6.112, p < 0.0001), which is not surprising since the two distributions do not even overlap. Thus, men with high paternity confidence are less likely to be incorrect in their assessment of paternity than men with low paternity confidence. In other words, men with high paternity confidence are more accurate in assessing paternity.
The median nonpaternity of men whose paternity confidence is unknown is 16.7% (range: 2 - 32). This is significantly greater than the high paternity confidence sample (Wilcoxon sign-rank test, z = -4.349, p < 0.0001), and significantly lower than the low paternity confidence sample (Wilcoxon sign- rank test, z = 3.528, p = 0.0004).
When the high and unknown paternity confidence samples are combined, the median nonpaternity is 3.9% (range: 0.4 R 32). This is significantly less than median nonpaternity for men with low paternity confidence (Wilcoxon sign-rank test, z = -6.053, p < 0.0001). Thus, men with low paternity confidence are the least accurate in assessing actual paternity.
In other words, interpreting the results rather cavalierly, if you're fairly confident that the child is yours, it is probably yours (although, interestingly, nearly 1 out of 50 such men are still mistaken). If you've got suspicions, then the probability that it's not yours is nearly one out of three. Intuitively, this makes sense: men don't just doubt for no reason. If you have reason to doubt, then it's probably with some justification.
Anyway, the 5-10% number seems to be a bit of an overestimate; the actual number from the most comprehensive survey seems to be in the 2-4% range. Still disturbingly high, in my opinion.