Well it's not a great study you can look at all the players named in the Mitchell Report and see that most of them stayed pretty crappy. There are definitely some players who took steroids and it correlated with their career years, but correlation does not imply causation. I can see steroids having some minimal impact, I just think it is totally overblown and unproven. Even adding 15-20 HR is a huge effect unsupported by any credible evidence. I also think there was just something going on in the league during the years that home runs peaked (whether expansion, or pitching quality, or a juiced ball, whatever). For example, who was third in MLB in 1998 behind McGwire and Sosa? Ken Griffey Jr. with a career-best 56 HR. I have never seen anyone accuse Griffey of taking steroids, and he had a major power spike in the same years.