In short, you're less likely to be attracted to someone of your preferred gender if you both grew up together. The Westermarck Effect is commonly brought up in the midst of
shipping arguments shipping discussions where incest, cousins, and childhood friends are involved. A form of reverse sexual imprinting, the Westermarck Effect serves to suppress inbreeding in humans, working in opposition to the theorized genetic sexual attraction phenomenon. The following factors are known to be involved:
- Proximity: The observed effect applies to children raised in close contact.
- Age: The critical period for reverse sexual imprinting ends by age 6 or 7.
- Age Difference: If more than eight years apart, the effect is greatly diminished for the younger participant.
- Genetics: Lack of blood relation makes no difference.
- Gender: Women and girls may be more sensitive to the effect than men and boys.