Genetic sexual attraction

You're 40, happily married - and then you meet your long-lost brother and fall passionately in love. This isn't fiction; in the age of the sperm donor, it's a growing reality: 50% of reunions between siblings, or parents and offspring, separated at birth result in obsessive emotions. Last month, a former police officer was convicted of incest with his half-sister - but should we criminalise a bond hardwired into our psychology? Alix Kirsta talks to those who have suffered the torment of 'genetic sexual attraction'.