In one of the most famous scent studies ever conducted, researchers asked women to sniff unwashed T-shirts worn by men—and rate them based on how attractive the scent was.

The result? Women consistently preferred the smell of men whose immune system genes—specifically the major histocompatibility complex (MHC)—were different from their own.

This matters because MHC genes play a big role in immune system function. From an evolutionary standpoint, pairing up with someone who has a different MHC profile could give future offspring stronger immunity.

And yes—this all happened without anyone knowing anything about the men who wore the shirts. No photos. No names. Just sweat.

Evidence suggests that your nose might be doing some quiet matchmaking long before your brain gets involved.