Henry   Heather        
    |   |        
Helen — Cindy — Carlos — Olivia — Harold
    |   |        
    Harry   Hector        

Most people have (on average) fewer friends than their friends have.

It’s one of those things that are obvious once you think about it, but not before. You just need to realize that you are more likely to be friends with someone who has many friends, like Cindy, Carlos, or Olivia in this example.

  Friends   Average friends of friends
Henry 1 Cindy(4) 4
Heather 1 Carlos(4) 4
Helen 1 Cindy(4) 4
Cindy 4 Henry(1), Carlos(4), Harry(1), Helen(1) (1+4+1+1)/4=1.75
Carlos 4 Heather(1), Olivia(2), Hector(1), Cindy(4) (1+2+1+4)/4=2
Olivia 2 Harold(1), Carlos(4) (1+4)/2=2.5
Harold 1 Olivia(2) 2
Harry 1 Cindy(4) 4
Hector 1 Carlos(4) 4
Average 1.78   3.14

One would think some ancient mathematician or philosopher could come up with this idea. Yet there is no trace of it until 1991, when Scott L. Feld published his article in the American Journal of Sociology.

Source: Friendship paradox