Who are They?

Being a new mom to my almost 6 month old first child, I can easily say this is a new experience for me. Now I think I have purchased just about every parenting book, pamphlet or snippet out there for us new nervous parents. We also have the well meaning relatives that are ready, willing and able to lend their advice or experience to us at the drop of a hat. The biggest person we hear from though is They. Who is they you ask. They is that all knowing creature that all knowledge comes from. For example: They say you should put the baby to sleep on her back. They say that you should breastfeed exclusively for 1 year. They say that she should be rolling over now. Who are They and why do They know so much. I've found myself saying "Well that's what They say" more in the past 5 months than I've ever said it before (that I've noticed). Why? Is it a reasonable answer to give someone when they ask you a question. That's what They say.
I want to know how you get a degree to be a They, is it a four year or two year college degree? Had I known there was so much money and knowledge to be made in being a They, I would have looked into it years ago. Then someone would be quoting me!