Its official, scientists have shown that seeing a pretty woman, makes men temporarily act like an idiot
http://www.newscientist.com/news/news.jsp?id=ns99994469Pretty women scramble men's ability to assess the future
Biologists have long known that animals prefer immediate rewards to greater ones in the future. This process, known as "discounting the future", is found in humans too and is fundamental to many economic models.
Male students, when shown pictures of pretty women, were more likely to opt for short-term economic gain than wait for a better reward in the future.
Wilson and Daly found that male students shown the pictures of averagely attractive women showed exponential discounting of the future value of the reward. This indicated that they had made a rational decision. When male students were shown pictures of pretty women, they discounted the future value of the reward in an "irrational" way - they would opt for the smaller amount of money available the next day rather than wait for a much bigger reward.
Women, by contrast, made equally rational decisions whether they had been shown pictures of handsome men or those of average attractiveness.
Theres some talk about evolutionary biology, but I think its much more easily accounted for as a bloodflow problem.