Mainly through direct observation. Among my own circle of friends, it's surprising how neatly we (well, those of us who use either language) divide into those categories. But you can see the pattern in job ads, discussions on mailing lists and in the domain-specific libraries for each language too. As many people got into Python through NumPy as a MATLAB alternative as got into Ruby via Rails, I'll bet.

Like I say, this isn't a good thing or a bad thing, it's just interesting.

Your comment made me realize that the phrase direct observation sounds so much better than anecdotal evidence.

You'll notice I did say my experience :-) I'm not trying to "win" an argument here, which would require "evidence", just pointing out that there are more important things in language selection than the features/characteristics/quirks of the language itself, especially given there's not much to call between Ruby and Python.

But, y'know, you could try quantifying it if you like, by looking at the libraries available for each language, and looking at how many jobs are "Ruby" as their main skill and how many are "10 years of whatever + Python".

Certainly not a bad or good thing, it was just interesting to me because I've never noticed the same trend from interacting with the Python community in Chicago and at PyCon.