Ho Effect

once 2 people of opposite gender converge on a single point, a new person (usually female, but sometimes male) will join the room, and thus the balance tipped. With this new change in gender, more of the dominant gender flock, trying to get the attention of the lone male (or female)
Once there's a 3 - 5 to 1 ratio, another male will join, and thus the ho effect has hit full circle.

Stage 1:Guy 1 & Girl 1 hang out at the mall.
Stage 2:Girl 2 joins them.
Stage 3:Girl 3 sees that girl 1 & 2 are getting all the attention from guy 1, and joins their group.
Final Stage:Guy 2 sees guy 1 hogging all the attention, and joins to steal some girls for himself.
Thus the Ho Effect