Are volunteers better fighters?

It seems so:

A voluntary army’s quality exceeds or falls below a drafted army’s average quality depending on whether selection is advantageous or adverse. Using a collection of data sets that cover the majority of the US Army soldiers during World War II, we test for adverse selection into the army. Rather, we find advantageous selection: volunteers and drafted men showed no significant difference in fatalities, but volunteers earned distinguished awards at a higher rate than drafted men, particularly after the attack on Pearl Harbor. Analyses at the level of units concur with our findings based on enlistment records.

That is from a newly published article by Javier A. Birchenall and Thomas G. Koch, via Robin Hanson.