Why do most pop groups have an odd number of members?

Especially kpop, but most pop groups have an odd number. cat Dolls have 5, Sugarbabes & Destiny's Child have 3, Super Junior 13, TVXQ, SHINee, Backstreet Boys, NKotB, f(x) have 5, SNSD has 9. I know there are groups that have an even number, but most have an odd. Is it just to have a distinguishable leader?