Steve Stewart-Williams argues against the view that incest taboo is a Western phenomenon, arguing that while brother-sister marriage was reported in a diverse range of cultures such Egyptian, Incan, and Hawaiian cultures, it was not a culture-wide phenomenon, being largely restricted to the upper classes. Stewart-Williams argues that these marriages were largely political (their function being to keep power and wealth concentrated in the family) and there is no evidence the siblings were attracted to each other and there is in fact some evidence against it (for example, Cleopatra married two of her brothers but did not have children with them, only having children with unrelated lovers). Stewart-Williams suggests that this was therefore simply a case of social pressure overriding anti-incest instincts. Stewart-Williams also observes that anti-incest behaviour has been observed in other animals and even many plant species (many plants could self-pollinate but have mechanisms that prevent them from doing so).