Incest
Incest is now regarded as bad for the genetic health of the offspring, and it is often forbidden by present-day religions and laws. The fundamental reason has to do with genetics. Incestual societies will have a high rate of birth defects, significant enough for it to be noticed even when little was known about the cause. To some extent this is a surmise, because ancient societies had no knowledge of genetics as such. But they must have noticed some connection between birth defects and the relationship between parents. We think this because widespread bans on closely related persons marrying came long before the understanding of modern genetics.