Females in socially monogamous species require a period of courtship before
they start to reproduce. When female reproductive success depends on male
assistance, such 'coy' behaviour might have evolved in response to male phi
landering. In this paper, we use a dynamic optimization model to demonstrat
e that female coyness can be maintained during evolution (provided that a f
emale is able to reject a male and find a new one) if two conditions are me
t. First, both faithful and philandering males must be present. Second, a f
emale must be able to learn more about a male's intention by spending time
with him. In our model the optimal duration of coyness is a trade-off betwe
en the benefit of waiting and gaining information about the male partner an
d the cost of postponing the onset of reproduction. This trade-off is influ
enced by several factors, such as the time needed to find a male, the level
of male philandering and the possibility of detecting these males, the dec
line of the reproductive success with time and the disadvantage of reproduc
ing with a philandering male.