Nobody knows for sure. But some scientists think that kissing might help us assess a partner’s immune system. When people kiss, they come close enough to sample the scent of each other’s bodies. And that scent may contain clues to immune system compatibility.
But why is that important? Because parents with complementary immune systems have children with a stronger ability to resist disease. Now I see, children that are better at resisting disease are more likely to survive to have their own children. Evolution will favor those who choose partners with compatible immune systems over those who don’t, even if they don’t even know that they are doing it.
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