It used to be that we couldn't even talk about incest, except in horrified whispers and puerile middle-school playground chants. (Remember this one: "Incest is best, put your sister to the test!")
Now it's water-cooler chatter. On the extreme criminal end, there's Josef Fritzl of Austria, who held his daughter captive for 24 years and fathered seven children with her in the basement of the family home.
Less extreme but still stomach-turning is Warren Jeffs of Utah, now in prison for forcing a girl, 14, to marry her cousin, 19.
But there's also the quiet stories of everyday people who happen to be blood-related and happen to fall in love. Some of these couples are speaking out, and some hypothesize that when it comes to siblings who have grown up in separate households, it's sort of natural.
The phenomenon is called genetic sexual attraction (GSA), and some researchers believe it's related to what's called imprinting, or a child's normal response to the face of the parent or caretaker of the opposite sex.
At puberty, it's thought that many adolescents will be unconsciously drawn to those with similar physical characteristics, as his or her parent.
ABC's "Good Morning America" recently featured Danielle Heaney and Nick Cameron of Scotland, who have the same mother but different fathers. The two told GMA that they felt instantly attracted to each other when they first met as adults; they'd only met once as children. They have been together ever since.
"There's no real data on this," says Jonathan Turner of University of California at Riverside. "Most likely it's an indirect mechanism, not straight genetic-genetic attraction. People with shared interests and personality traits tend to like each other."
But when it comes to siblings, cousins and even unrelated children, such as those who grow up on a kibbutz, research has shown that an opposite phenomenon sets in. It's called the Westermarck effect.
And unlike the somewhat debatable GSA, researchers are quite sure that the Westermarck effect is biologically — not psychologically — based.
In a nutshell, kids who grow up in close proximity — sharing bath and bedrooms, rolling around on the kitchen floor, wrestling and fighting, even hugging and kissing — will develop a strong "positive" sexual aversion to each other when puberty sets in. It's the feeling most of us get when we imagine sex with a sibling, a visceral "yuck."
Ironically, says Turner, in families and cultures where brothers and sisters are separated at an early age, the Westermarck effect never gets a chance to set in, thereby increasing the chances that siblings will develop an undesirable attraction later in life.
As an aside, parents shouldn't worry too much if they catch their prepubescent children playing "doctor." It's common, says Turner, and almost always stops at puberty when Westermarck sets in.
"It's hard to get data on incest," he says. "It's probably more common than once thought. But from the data I've seen, and it's really lousy data, it shows that for the most part either the kids were raised apart, or the family is abusive and dysfunctional."
One of the most common scenarios, he says, is a dynamic involving an abusive father and an angry son, who takes out his aggression on his younger sister.