text: why is incest immoral (and, i think, illegal)?   incest (sexual relations with a close relative) is a social taboo and laws may have been passed against it to prevent inbreeding (breeding and having children with a close relative) which can result in;  ...reduced fertility  ...increased congenital disorders ...fluctuating facial asymmetry ...lower birth rate ...higher infant mortality ...slower growth rate ...smaller adult size ...loss of immune system function.  can't have civilization if the population becomes lower in numbers, more susceptible to disease and have a decrease in physical and mental capability.