Men's minds may be so rattled by the sight of a pretty woman that they behave irrationally, Canadian psychologists have shown.

Scientists know animals prefer short-term gains to longer-term ones, even if the future payoff is larger. Advertisements featuring attractive women take advantage of the effect in people.

Researchers at McMaster University in Hamilton, Ont., designed an experiment to investigate sex-related "irrational discounting."

Actress Julia Roberts starred in the movie Pretty Woman

Psychologists Margo Wilson and Martin Daly asked 209 students to look at pictures of the opposite sex from the website "Hot or Not."

The participants were then asked if they'd prefer receiving an average of $19 immediately or waiting for $25 at some future time.

After eyeballing faces of women ranked as pretty on the website, men were more likely to want the immediate payment rather than hang on for a bigger bonus.

Women's choices, though, were unaffected by photos of handsome men.

The researchers suggest men may want money immediately to impress the ladies.

The study appears in this week's online issue of Proceedings of the Royal Society London B.