Why are there so many co-authored papers?

A loyal MR reader asks:

I’m reading (grr) a lot of academic papers lately and, to keep myself awake, have wondered: why do they almost always have more than one author? Is it like cops in New York City–they’ve got everyone persuaded it’s too dangerous to go alone? Is there some networking benefit, professional or psychological? Does it just enable everyone to claim more publications? Has anyone studied which fields have the highest and lowest average number of authors per paper?

I thought you could blog something interesting on this. I might add that the papers couldn’t be any duller, and I wonder if committee authorship plays a role in this as well.

I believe that co-authored papers are correlated with:

1. The existence of a laboratory

2. Senior scholars who generate funding and thus gains from trade

3. Empirical work, which tends to be more divisible than theory; co-authored papers are relatively rare in pure economic theory and in philosophy

Co-authored papers are becoming increasingly common in economics, also because the effort requirements for top publications have been rising.  In most cases a co-authored piece is worth at least 2/3 of a singly-authored piece, so the incentives for co-authorship are strong.  Here is an earlier post on co-authorship.