3 Unifiers in human interaction.?

I am trying to remember what three diffrent factors cause people to group together are, I know the first two are common needs and common fears, but I can't remember the third one. I want to say it is something like common goals or desires or intrest, but these all sound to much like common need, any help is... show more I am trying to remember what three diffrent factors cause people to group together are, I know the first two are common needs and common fears, but I can't remember the third one. I want to say it is something like common goals or desires or intrest, but these all sound to much like common need, any help is appreciated, but I would pefer a source to verify it.
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