Originally posted by Rufus Moses: Actually I think its more like the covergence of the need for sex and the need for money. i agree that Darwin's natural selection and Adam Smith's free trade in combination explain more of life than any other two models. but, no economist (except those that are still old-fashioned marxists) believes that economics can explain all social interactions. and, although some evolutionary biologists are celebrating the end of their long twilight under the shadow of sociology by making claims of explanatory universality that are clearly excessive, many more sober biologists point out that genetic variation (the essence of sex's scientific function) can only explain so much. TS Eliot once quipped that life is "birth, death, and reproduction" (thus, reducing everything down to sex -- without even including the money). and yet, of course, TS Eliot's life and work prove otherwise. Which is not to say the transaction can't be fun, good natured, well meaning, a positive nurturing experience for both, blah blah woof woof... whoa -- "nuturing"? 'nuturance' is something that neither genetic biology nor optimization economics can explain -- or even describe. But it really is sex and money. we all agree on that. and, those of us who have been around long enough to have the morning dew cleared from our eyes are aware that the same thing could be said about marriage. but. the question remains: is that all there is for all couples at all times? if it were, then at the most basic level, the mileage would never 'vary' for a given sum of money. but, on the contrary, it always seems to vary. and neither biology nor economics, nor their combination, can begin to explain why that is.