Legal impediments may include: the prohibition of incestuous marriage; the marriage of persons afflicted with imbecility, epilepsy, venereal disease or tuberculosis; child marriage; or mixed marriage. In countries where polygamy is against the law, the existence of a legally viable marriage of one of the parties prevents him or her from remarrying.
The prohibition of incestuous marriage is widespread, as is that of child marriage. The legal prohibition of marriage of persons afflicted with imbecility, epilepsy, venereal disease or tuberculosis occurs in the USA, but is rarely enforced since there must be written proof of these diseases. In the UK, mental disorder or impotence are legally accepted reasons for refusing marriage, but if they are discovered after marriage they will not invalidate it.