Although dating or marrying close relatives is typically frowned upon in the society, it did not have a scientific basis till recent studies. Apparently, these studies show that mating with a close relative also known as inbreeding reduces the evolutionary fitness of the offspring. It might even cause reproductive issues in the later generations.
David Clark of the University of Edinburgh and his colleagues collected data of over a million people from culturally diverse population to get a better idea of the problems caused by inbreeding. When the inbred progeny were compared to their outbred peers, it was determined that the inbred progeny seemed to have 0.11 fewer children and were more likely to be childless as well. All these factors indicate a huge impact on the reproductive abilities as a result of inbreeding. Even the progeny who is born to the inbred generation seems to be lacking behind their peers, as their height and weight were noticeable lesser. The children were also determined to be a lesser educated maybe because of a slightly diminished intellectual capacity.
In fact, David Clark of the University of Edinburgh and his colleagues determine that inbred progeny are more prone to genetic diseases, especially if both parents are carriers of the trait. This study is proof of why it is a better idea to marry or procreate with people outside of your family circle.