High-profile paternity suits point to a trendy obsession for the ultimate proof of biological fatherhood.

But to evolutionary psychologists, they're just a recent and hi-tech twist on age-old anxieties.

Since hunter-gatherer times, men have relied not on DNA swabs but on a little-understood calculus of physical resemblance to decide whether to invest in little Emma or Ethan. In the infant's upper face and eyes, the skeptical pater familias looks for clues.

Comedian Chris Rock probably went through a similar mental process after a Georgia woman claimed he fathered of a child she had 13 years ago. Ditto Larry Birkhead and Howard K. Stern when they first saw Anna Nicole Smith's baby daughter, Dannielynn, born last September. Birkhead, after DNA testing, was determined the biological dad and won custody of his little look-alike following Smith's death last February.

The latest study, done early this year by Brock University psychology professor Anthony Volk, show cues of genetic relatedness are more important to men than women. He showed photos of infants' faces to male and female subjects, and asked them to make hypothetical adoption choices. In the journal Evolutionary Psychology, Volk reported that men reacted more positively to children with facial traits resembling them, while women's decisions were influenced more by healthy looks.

Research by Volk and other psychologists over the past five years has refined the parental "investment" model that was first put forward by American evolutionary biologist Robert Trivers in 1972.

Trivers held that parents prefer to invest in offspring that are biologically theirs. But males, unlike females, aren't obliged to provide care. It followed, researchers theorized, that males literally look for their genes and that things like similar eye colour, features or the shape of the face can serve as cues of resemblance.A trio of U.S. studies in the past decade have found it is not uncommon for stepchildren to receive significantly less time, financial support, and emotional resources than biological children. As well, the less the male thinks the child resembles him, the worse he treats the child. The three studies determined infanticide and spousal murder are often committed by men who learn a child isn't theirs, or who come to believe it.

Males may have acquired their skill at discerning resemblance to deal with "extra-pair paternity" – sex outside of a monogamous relationship – explains Steven Platek, the author of several studies in this field and a lecturer in evolutionary psychology at the University of Liverpool, in Britain. The extra-curricular coupling, particularly among animals, helps females ensure "good gene" acquisition.

It's not clear who started this co-evolutionary tit-for-tat, but female infidelity represents a significant "cost" to men, Platek says in a phone interview from the U.K.

Non-paternity estimates are very much in dispute. University of Oklahoma anthropologist Kermyt Anderson concluded in a 2006 study that between 1.7 and 3.3 per cent of men who have "high paternity confidence" are not the biological fathers of their supposed children.

Yet those numbers are much lower than the oft-cited "non-paternity" rate of 10 per cent offered by one researcher in 2002 and Platek suggests the rates may be even higher. He notes uncertainty around female fidelity means "your line of genes could actually be (deleted from) the population if you're raising offspring that aren't related to you," he says.In a 2004 study, Platek and colleagues from two other U.S. universities scanned the brains of men and women during a hypothetical adoption exercise. They found that males activated an area of their brains responsible for complex decision-making, as though they were thinking about whether, or not, to invest in a child.

Volk says his studies suggest that "women, involved in the care of the baby, are most concerned with `Is this baby healthy? Am I going to have to spend a lot time with this baby?' and `Is this baby cute: will it be successful?"

Which all contributes to men being preoccupied with resemblance – and, perhaps, a nagging fear of cuckoldry.

Despite all of this, there's no actual proof that males are any better than females at assessing resemblance.

In fact, facial similarities are relatively tough to spot, and some researchers believe that's by design.

Volk says he's tempted to believe that the "anonymity" of babies' faces is due to a desire to "hide their identity" in the 24-month period in which they are most at risk of abuse and abandonment.

And yet males aren't "locked into" their bias towards resemblance, Volk is careful to stress. A significant number of fathers raise children that are not theirs – and raise them successfully. Such fathers have often been ridiculed in popular culture, although there have been sympathetic portrayals in recent films, including the ailing patriarch in last year's Oscar-nominated Danish film After the Wedding, and R.H. Thomson's character in Who Loves the Sun by Montreal filmmaker Matt Bissonnette.

Then again, it's possible these males were heavily influenced by what researchers call the "social mirror" – people being told the child looks like them. Maternal relatives are reportedly eight times more likely to say the baby looks more like dad than the mother. This can be seen as an effort to persuade men "that resemblance is there, even if it's not," Volk says.

The tactic can be very effective. Studies have found that, when presented with photos of strangers and told there are resemblances, people are biased to look for resemblances. But the strategy is delicate and can backfire. Some statements "may raise suspicions and change his behaviour," as opposed to the casual "Oh, he's got your eyes," Platek quips.

In addition to facial characteristics, there's some evidence to suggest that males even look for similar upper torso shapes, expressions of youth, or personality traits "at least in the mind of the father," Platek says.

"Our evolutionary history has designed us to think along those lines at a subconscious level.

"Nowadays, with DNA paternity testing, and the media, it's become more of a conscious mechanism, sort of playing off our evolutionary history," adds Platek.