[R]esearch shows men who spend even a few minutes in the company of an attractive woman perform less well in tests designed to measure brain function than those who chat to someone they do not find attractive.  Researchers who carried out the study, published in the Journal of Experimental and Social Psychology, think the reason may be that men use up so much of their brain function or 'cognitive resources' trying to impress beautiful women, they have little left for other tasks.
. . . .
  Each [male subjext] performed a standard memory test where they had to observe a stream of letters and say, as fast as possible, if each one was the same as the one before last.
  The volunteers then spent seven minutes chatting to male or female members of the research team before repeating the test.
  The results showed men were slower and less accurate after trying to impress the women. The more they fancied them, the worse their score.
  But when the task was repeated with a group of female volunteers, they did not get the same results. Memory scores stayed the same, whether they had chatted to a man or a woman.