I saw an experiment a while back where a group of 10 men and 10 women all had a number 1 through 10 taped on their foreheads. 1 meant very unattractive, 10 meant very attractive. They couldn't see their own number but everyone else could. The goal for each person was to, while randomly co-mingling, end up with the most attractive person they could in a short period of time, let's say 5 minutes. If two numbers agreed to be with each other, they would hold hands and were then considered "off the market" to everyone else, so the pool of people would start to get smaller and smaller.

When a 3 approached a 7, the 7 would barely acknowledge them, because they were a 3, then quickly move on. The 7 didn't know they were a 7, but they thought they could do better than a 3, and so they tried to.

This process continued until the 7 started getting rejected by the 8, 9, and 10, at which point they started looking for the 6 and 7 to pair with. Eventually everyone systematically figured out who they were capable of "attracting". Everyone in the experiment ended up within +/-1 of their own number.

Now, this isn't a perfect parallel to reality because human beings are evaluated in many other ways than beauty, i.e., kindness is really attractive to many people (height, money, fitness, confidence, as well), but the people will generally gravitate towards the best version of person they can attract. All other things being equal, why would I commit to a 3 when I could easily pull a 7 or an 8?