A recent piece by Andy Leipold (Illinois, Law) and Hossein Abbasi (Illinois, Ph.D. cand., econ.) will certainly interest many criminal law folks. Their paper, at 59:2 Vanderbilt Law Review 349-404 (2006) (or here), empirically tests two widely held assumptions. First, that criminal defendants facing multiple charges in a single trial are worse off than defendants facing several trials of one count each. Second, that a defendant joined at trial with co-defendants is worse off than a defendant standing trial alone.
What they find is that, "defendants who face multiple counts are roughly 10% more likely to be convicted of the most serious charge than a defendant who stands trial on a single count." Also, that "joining co-defendants in a single trial had virtually no impact on the likelihood of conviction."