"The right question is, What is it about certain/many good developers that leads them to program in their spare time?"
I think there's another way to look at this.
The fact is, programmers who program in their spare time are usually better than average (). That doesn't mean a programmer can't be just as good if he/she doesn't* program in their spare time. It doesn't even mean that programming in their spare time helps these programmers get better, or that better programmers tend to program in their spare time. It's the other way around - programmers who program in their spare time tend to be better (and one direction being true doesn't mean the other direction is true).
The only reason people care about whether others program in their spare time, is because this correlation exists. If I know someone programs in their spare time, that's a positive signal that they are a good programmer. And it's a perfectly legitimate shortcut, when interviewing someone, to see whether they're good. People take offense at the suggestion that they're "worse" because they don't program in their spare time, when in reality, it's just a shortcut.
* I think most people will agree, at least in terms of averages. Nothing about any particular programmer.
I think there's another way to look at this.
The fact is, programmers who program in their spare time are usually better than average (). That doesn't mean a programmer can't be just as good if he/she doesn't* program in their spare time. It doesn't even mean that programming in their spare time helps these programmers get better, or that better programmers tend to program in their spare time. It's the other way around - programmers who program in their spare time tend to be better (and one direction being true doesn't mean the other direction is true).
The only reason people care about whether others program in their spare time, is because this correlation exists. If I know someone programs in their spare time, that's a positive signal that they are a good programmer. And it's a perfectly legitimate shortcut, when interviewing someone, to see whether they're good. People take offense at the suggestion that they're "worse" because they don't program in their spare time, when in reality, it's just a shortcut.
* I think most people will agree, at least in terms of averages. Nothing about any particular programmer.