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To be more specific, why do so many good backgammon/chess/bridge players also make good poker players?

[/ QUOTE ]Some people are simply good at games. I suspect that someone who is good at poker or bridge could become good at gin rummy, for instance.

There don't seem to be many who are great at two (or more) of these games. Though I suppose it depends on your criteria for "great". There's no present day Oswald Jacoby, for instance. And, it's been a while since Billy Eisenberg was at the top of bridge and backgammon.