From a link someone provided above it seems legit.
the motorcycle used 28% less fuel than the comparable decade car and emitted 30% fewer carbon dioxide emissions, but it emitted 416% more hydrocarbons, 3,220% more oxides of nitrogen and 8,065% more carbon monoxide. The MythBusters’ conclusion: ‘At best, it’s a wash. Motorcycles are just as bad for the environment as cars,’ Savage said on the show. ‘At worst, they’re far worse.’
I recognize it's not an official study, but seems to be more concrete than anecdotal evidence and changed my perspective some. The article also quotes a researcher at UC Riverside which makes more sense on what's going on
‘We’ve been working to clean up passenger vehicles since the ‘70s,’ said Kent Johnson, who’s on the research faculty at UC Riverside and is director of its emissions lab, where the MythBusters’ numbers were analyzed. ‘We’ve been putting on catalytic converters and sensors to improve their ability to control emissions. We didn’t start doing that on motorcycles until the 2000s. It just shows you how far we’ve taken passenger vehicles and how difficult it is to do the same thing with motorcycles. First of all, there’s no room. And the incremental cost might double the price of a bike.’