In brief

Is it love — or just stress hormones?

People in all societies and cultures fall in love. The common ingredients include the sensation of being in an altered mental state, intrusive thoughts and images of the beloved, and changes in behavior aimed at getting a reciprocal response.

The head-over-heels phenomenon is so universal that some researchers say it may have been programmed into us by evolution. The theory is that offspring may be more likely to grow up and reproduce if they have two adults looking after them, and that falling in love makes such couplings more likely. Of course, it's also true that love can be fleeting and may not lead to stable relationships and cozy childrearing. In any event, this theory does a nice twist to our usual ideas about the survival of the fittest. If nature loves a lover, maybe it isn't quite so red in tooth and claw after all.