Complexities of Sexual Selection -- Avoiding Incest

Another example of how environmental factors affect brain chemistry which affects sexual selection preferences.

Science & Technology at Scientific American.com: Evolving a Mechanism to Avoid Sex with Siblings -- Evolutionary psychologists claim humans evolved a detector for avoiding sex with close kin: "Whether subjects directly saw siblings as newborns or simply shared the growing-up process, they both behaved more altruistically toward these siblings and felt a stronger aversion to any sort of sexual contact with them. "