Why does incest cause defects in organisms? If humans have sex with other humans that are related the baby will most likely have a defect. But If you have sex with someone who you are not so closely related to your chances of having a defect are less. Why is that? Humans had to have started out sometime in the past, right? How did they reproduce without having defects. We know that population increases (in our world today) so it had to have had a beginning. How did the earliest people reproduce with each other if there wasn't a lot of them to choose from like today? I made an identical thread. Post replies in this one. I didn't mean to.