Most of us know that incest is wrong, either through cultural conditioning or what appear to be innate evolutionary cues, yet some humans can’t seem to resist the urge to have sex with their relatives. In fact, since the DNA of every living human is 99.9 percent the same, it may be much more common than we think. Still, that doesn’t mean incest can be acceptable in any case. As we will see, breeding with a close relative can result in some dire consequences.
Even if you believe that two consenting adults should be able to have sex with whomever they please, there’s no escaping the fact that reproducing with a close relative has some serious drawbacks. When we mate with someone with a completely different gene pool, our chance of passing on recessive genes is only 50 percent. Many times, those genes remain dormant, but when people who share a large number of genes breed, the chance of passing on conditions known as autosomal recessive disorders—conditions that are inherited through recessive genes, including cystic fibrosis, sickle cell anemia, and albinism—increases significantly.
Other side effects of inbreeding include the increased risk of infertility, birth defects like cleft palates, heart conditions, facial asymmetry, low birth weight, slow growth rate, and neonatal mortality. One study found that 40 percent of children whose parents were first-degree relatives were born with autosomal recessive disorders, congenital physical malformations, or severe intellectual deficits. According to the study, when first-degree relatives mate, the risk of their child suffering from an early death, serious birth defect, or mental disability increases to almost 50 percent.