At closing time, everyone seems more attractive.
In one study*, 220 bar patrons were asked to rate the attractiveness of other people in the bar at varying times of the night. At 9PM, female bar patrons received an average attractiveness rating of 5.5 out of 10, while men were rated a 5.0. By midnight, the average attractiveness rating for women had increased to over 6.5, and men's attractiveness had reached 5.5. This effect was still apparent when the results were controlled for amount of alcohol consumed (i.e. maybe it wasn't beer goggles after all).
What does it mean? One interpretation suggests that the perceived increase of attractiveness acts as a means to lower the standards of the as yet unpaired bar patrons. As a result, the remaining people might be more likely to have sex, which would give them an increased chance of propagating their genes into the next generation.
The gender difference is also telling. In this study, women were asked to rate male attractiveness, and men were asked to rate female attractiveness. The men in this subject pool displayed a much greater increase in how attractive they perceived the opposite sex. This supports the idea that, as the sex that must invest more energy in raising offspring, women tend to be more interested in finding a quality mate than lots of mates. If kangaroos hung out in bars, they would probably display a similar phenomenon. In contrast, even if fish hung out in bars, they, for the most part, would show no sex differences in attractiveness rating increase, because most fish have no sex differences in amount of parental care.
*Gladue, B.A., and Delaney, J.J. (1990). Gender differences in perception of attractiveness of men and women in bars. Personality and Social Psychology Bulliten, 16, 378-391.
Additional resources: Nida, S.A., and Koon, J. (1983). They get better looking at closing time around here, too. Psychological Reports, 52, 657-658.
Pennebaker, J.W., Dyer, M.A., Caulkins, R.S., Litowixz, D.L., Ackerman, P.L., and Anderson, D.B. (1979). Don't the girls get prettier at closing time: A country and western application to psychology. Personality and Social Psychology Bulliten, 5, 122-125.