Depends on two variables--Supply of beer and/or supply of money. A third variable might come into play depending upon opening hours for beer sales. A fourth variable would be type of beer.

Generally, I drink the last half fastest unless there's a deficiency in the first two variables. In the days when I partied into the early morn, the last beer would be finished real slow. The "koolaide" beers like Bud, Miller, Busch, etc., I olny like real cold, so they go down fast before they cam warm and become like Coors at any temp. But then, I'm certainly not average.

I suspect there is no empirical evidence per se, but that the second half is usually consumed faster.
a)it's not as cold because its been sitting there longer
b)the mind is that much more impaired so there is a positive feedback mechanism to have another (at least on the first few-negative feedback after more than 3-4)
c)it 'looks' more asthetic to have a fuller as opposed to less full glass.

Would that oil were so simple...