In my totally unscientific opinion, I believe most are converts from religious backgrounds.
I think religion's portrayal of God has made it's fair share of converts. The one's that I have talked with have been significantly affected by what they were told the bible said. I must admit, after have read the bible a number of times, the church's portrayal of God is very disappointing.
I'm an atheist and everyone In my family is a Christian... I study science and there are many reasons for me to not believe in a god... I'm only 15 and my mom knows my take on religion, she's not happy but hasn't said a word because I always ask a question to stump her :) Christians are afraid of questions like that and have you always noticed you get the same bullsh!t answer? :)
I have a friend who claims to be an atheist, I think she's doing it to rebel, not because of research...
I've always had atheist beliefs, but my parents never out right told us kids if God was real and what religion we are (because we don't have one) I've aquired these beliefs on my own
"Of the atheists in the world, are more raised atheist or do more convert to atheism?"
I don't believe anyone ever converted to atheism but rather "De-converted" from religion. We are ALL born without any knowledge or belief in God/Gods . . . That is something taught to you via indoctrination. Our natural default is one sans God/Gods. I was once a Born Again Christian, indoctrinated from early childhood, and when seeking further information about the Bible, and religions where the Bible was the center, I found no basis for a belief in ANY God/Gods, and further refute the un-validated Claim in the first place.
As I sought out a way to raise my own children with the benefit of no "Enforced Religion": But knowledge of religion, both children came to the conclusion of no belief in God/Gods. My oldest actually considered himself an Evolutionary Theist for a while, before coming to his own conclusion of Atheism . . . I think as more and more Atheist marry and have children with both parties Atheist . . . Less and less children will be indoctrinated to believe in God/Gods.
People have converted to being atheists because of what has been done in the name of religion. The wars, hatred, crime and violence all in the name of religion.