When 61-year-old Australian John Deaves and his 39-year-old daughter Jenny announced this week that they had a child together, most people reacted with shock and disgust.
But while many would cite simple social unacceptability for their reactions, sociologists and sex experts say that aversion toward incestuous relationships goes far deeper than the violation of social norms.
In fact, research suggests that the vast majority of us are hard-wired for revulsion when it comes to the idea of sex between a father and daughter or other family members.
"When incest occurs, it is not only violating a cultural taboo, it is crossing ancient neurological wiring for avoidance," said Jonathan Turner, a distinguished professor of sociology at the University of California, Riverside.
Reason for Revulsion
Turner, who along with his wife, Alexandra Maryanski, wrote "Face to Face: Toward a Sociological Theory of Interpersonal Behavior," says that this "neurological wiring" may not have come about as a social construct, but rather as an evolutionary imperative that has existed for tens of thousands of years.
Evidence of such an inherent distaste for incest can be seen in studies on chimpanzees, our closest evolutionary relative. While chimps are known to be promiscuous -- arguably even more so than humans -- several studies have shown that they will also avoid having sex with other chimps with whom they can sense they are related.
And recent research has reinforced the idea that even on a subconscious level, the concept of incest is not an attractive one to most people. Such were the findings of a team of researchers at the University of California, Santa Barbara, who last year published a study in the journal Nature showing that siblings possess a hard-wired instinct to avoid sexual relations with one another.
"The old thinking was that Darwinism applied to humans physically, but not socially," said study co-author John Tooby, a professor of anthropology and co-director of the Center for Evolutionary Psychology at UCSB in a statement. "Now we see the evolution of a mechanism that finely regulates important aspects of human social behavior."
At the heart of this tendency to avoid sibling incest is a principle known as the Westermarck effect. For reasons not yet completely understood, boys and girls involved in frequent rough-and-tumble play in childhood tend to not be sexually attracted to one another later in life.
Though little research has been done that would suggest this effect also applies in father-daughter relationships, Turner believes that this is a distinct possibility -- in other words, fathers who spend time raising their daughters from infancy are probably less likely to develop a sexual attraction toward them later.
This evidence of an innate revulsion toward incest, coupled with the general avoidance of incest in most cultures around the world, suggests that the taboo may be nature's way of helping us avoid the multitude of problems that come with inbreeding -- which include rare genetic diseases and defects.
"The data are quite clear that the rates of biological problems are rather high for children of those who share 50 percent of their genes; so, people in the distant past could see quite clearly that inbreeding among close relatives is harmful," Turner said. "The result was the creation of a taboo."
And sex experts say this taboo has had a profound effect on societal norms of sexual behavior.
"Reproduction with related individuals creates more genetic abnormalities," said Eli Coleman, a professor and academic chair in sexual health at the University of Minnesota Medical School. "It seems the incest taboo has its evolutionary advantages. Social structures are set up within cultures, which preserve what has probably been known for centuries and discourage interfamilial relationships."
"Here is a case that the sociopsychological aspects of the incest taboo probably has at least part of its roots in a biological imperative -- that of probable genetic mutation," said sexuality expert David Greenfield, clinical director of The Healing Center, LLC in West Hartford, Conn.
When the Taboo Doesn't Work
But the incest taboo, at least in the recent case in Australia, isn't ironclad -- a fact that became readily apparent when the couple appeared on Australia's Nine television network's "60 Minutes" program to explain their unusual relationship.
Part of the reason for this, Turner says, could be linked to the fact that Deaves left the family home when his daughter was only an infant and that he did not see her for the next 30 years. Any Westermarck-like effect that could have existed between the father and daughter would therefore not have had the opportunity to develop.
Turner adds that compared to mother-son and brother-sister incest, father-daughter incest is actually more common.
But despite this fact, it does not mean that children of father-daughter incest are less likely than other children of incest to have genetic problems. Indeed, while the Deaves' new daughter is reportedly healthy, court documents reveal that the couple had another child in 2001 who died of congenital heart problems.
"It is very likely that this malformation was due to incest because they share 50 percent of their genes," Turner said.