Intro Ethics versus Regular Intro

In my current job, I teach a lot of sections of Introduction to Ethics, and a lot of sections of Intro to Philosophy. I've noticed a general trend in my teaching whereby my Intro Ethics students generally do almost a full letter grade better on average than my regular Intro students (e.g. a B+ to the Intro's B-). I don't think it's because my Ethics class is easier, because the pattern extends to individual test questions concerning material that I include in both classes, such as the meaning of the word 'valid.'

One possible explanation is that students who take Ethics rather than Regular Intro tend to have a prior interest in the subject matter, and are not taking it only to satisfy a requirement. Since interested students do better, ethics students do better. This suggests that I need to do a better job getting my intro students interested in philosophy.

I'm doubtful of this explanation for a couple of reasons. For one, my ethics class is pretty theoretical, and I'm not sure where an 18-year-old public-school-type person would pick up a prior interest in theoretical ethics. If my students are interested in ethics, they're interested in applied ethics. Second, I spend a bunch of time in intro on the theism/atheism debate, which I know many of my students have a prior interest in. But it's not obvious that this isn't it.

Another possible explanation is that I am more interested in my ethics class, and this is affecting my students. This is definitely possible. But I'm pretty interested in e.g. the theism/atheism debate and other topics I cover (although I do not publish in these areas, and it is easy for my students to discover my publication record. However, I doubt that many of my students take the initiative to discover the details of my publication record.). And I've talked to a few other people, LEMmings, who report the same phenomenon as me.

So I'm not sure what's going on. What say you, Smokers?

--Mr. Zero