(NEW YORK) -- Julie DeNeen was raised by her biological mother and a step-father who adopted her after DeNeen's birth father relinquished his legal rights. But she yearned for the father she never knew, wondering why he abandoned her.
"I had no picture and no contact with my biological father," said DeNeen, now 31 and married with three children in Clinton, Conn. "I hardly knew he existed."
At 13, DeNeen sought out and found her father, but after a few visits they grew apart.
"I viewed him as a strange relative," she said. "I wasn't prepared … and it was just awkward."
But in 2011, again obsessing over her family roots and wanting her children -- 8, 6 and 5 -- to know their grandfather, DeNeen wrote him a letter, telling him she was sorry "I had fallen off the planet" and that she loved him and wanted him to "be a father figure in my life."
So they reconnected, but what evolved was far from a healthy father-daughter relationship -- amidst her grief and longing, sexual sparks flew and it nearly destroyed her 10-year marriage and a fragile new bond with her biological father.
"I had this strange falling in love feeling, holding my Dad's hand," said DeNeen. "It wasn't like a daughter, it was like something else."
That something else was genetic sexual attraction or GSA.
Psychologists say that taboo is normally in place when family members grow up in close proximity by virtue of reverse sexual imprinting, or the Westermarck effect, which desensitizes them to later sexual attraction. Researchers hypothesize it evolved so biological relatives would not inbreed.
The phenomenon was first identified by Barbara Gonyo in the 1980s. She wrote a book, I'm His Mother, But He's Not My Son, that recounted her personal story of reuniting and having sexual feelings for a son whom she had placed for adoption when she was 16. Gonyo fell in love -- a byproduct of delayed bonding that would normally have taken place in infancy, had they not been separated by adoption.
GSA is "not incredibly common," but is seen among parents and adult children and between adult siblings, according to Susan Brancho Alvarado, an adoption therapist from Falls Church, Va.
And because of that, mental health experts are not experienced in helping patients. They often mistakenly confuse GSA with incest or sexual abuse, shaming adoptees.
Alvarado, who has treated four families with GSA, also blames the adoption process itself.
"It fuels the secrecy and builds up the fantasy about what the other family might be like," she said. "It is mitigated when you have open access to records and birth certificates and the family from infancy is included."
Copyright 2012 ABC News Radio