Sci-fi and crime: What's the connection?

So here's a question: What, other than money, might dispose a crime writer to turn to science fiction or vice versa? What features do the genres share, or is it just a matter of certain writers' simply liking to work in both forms?
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Late addition: B.V. Lawson's In Reference to Murder blog adds Stanislaw Lem to the roster of science-fiction/crime-fiction boundary jumpers with his novel The Chain of Chance.© Peter Rozovsky 2009
Labels: Isaac Asimov, John D. MacDonald, miscellaneous, Ray Bradbury, science fiction