For children who turn out not to be the offspring of their putative fathers, percentages as high as 10% are quoted as lab lore by people involved in genetic testing for inherited disorders. I have yet to find a peer-reviewed publication that addresses these statistics specifically (my keyword searches through the databases haven't been sharp enough so far, I'm afraid), but the figures are not insignificant.

Leaving aside the possibility of inadvertent consanguinity as conveyed by the article, this makes me wonder: are we a species where some males unknowingly look after the genes of other (perhaps more sexually assertive) males alongside their own? The answer is obviously yes, but how far in the past does this reproductive pattern go? How widespread is it among different societies? How relevant to the composition and evolution of our gene pool? And what about the role of the female of the species in this reproductive triangle?

In other words, is this an accident, or is it what we do, and why? To be sure, these themes have not gone unexplored. Here's an interesting example of (in my opinion) a relevant study:

Male facial attractiveness: evidence for hormone-mediated adaptive design

V.S. Johnston, R. Hagel, M. Franklin, B. Fink and K. Grammer

Evolution and Human Behavior, vol. 22, pp. 251-267 (2001)

Experimenters examining male facial attractiveness have concluded that the attractive male face is (1) an average male face, (2) a masculinized male face, or (3) a feminized male face. Others have proposed that symmetry, hormone markers, and the menstrual phase of the observer are important variables that influence male attractiveness. This study was designed to resolve these issues by examining the facial preferences of 42 female volunteers at two different phases of their menstrual cycle. Preferences were measured using a 40-s QuickTime movie (1200 frames) that was designed to systematically modify a facial image from an extreme male to an extreme female configuration. The results indicate that females exhibit (1) a preference for a male face on the masculine side of average, (2) a shift toward a more masculine male face preference during the high-risk phase of their menstrual cycle, and (3) no shift in other facial preferences. (...)

(In men, a more masculine face has been linked to higher male hormone levels, and there may be a relation between face symmetry and perceived health.)

Another consequence of non-biological fatherhood is the dubious accuracy of genealogical studies. Here, though, I'd like to quote from a study by Bryan Sykes, who found a significant correspondence between his surname and a cluster (haplotype) of variable genetic markers (microsatellites) on the Y chromosome, which is exclusively transmitted from father to son:

Surnames and the Y Chromosome

B. Sykes and C. Irven

American Journal of Human Genetics, vol. 66, pp. 1417-1419 (2000)

A randomly ascertained sample of males with the surname "Sykes" was typed with four Y-chromosome microsatellites. Almost half the sample shared the same Y-chromosome haplotype, which has not been observed in control samples either from the same geographic region or from the United Kingdom as a whole. This points to a single surname founder for extant Sykes males, even though written sources had predicted multiple origins. The distribution of other Sykes Y-chromosome haplotypes were not significantly different from those in controls and may be accounted for by the historical accumulation of nonpaternity during the past 700 years, in which case the average rate estimate is 1.3%/generation. If this pattern is reproduced with other surnames, it may have important forensic and genealogical applications.

By the way, matrilineal inheritance is traced on the DNA from mitochondria, mtDNA, cellular elements inherited from the mother that possess a small genome distinct from the main (nuclear) genome.

As a matter of curiosity, B. Sykes wrote a book with a catchy title, The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry and founded a startup company that proposes individual Y and mtDNA studies to the public. As of this writing, my curiosity has not overcome my reluctance to spend money, and I ignore which daughter of Eve conceived by her husband or another man to give rise to my maternal lineage.