I don't know. I think it's probably true, but I don't know . The reason I think it's probably true is that we're not born with equal opportunity for material happiness. It's that simple. We're not born equal.
Why are we not born equal? To me it makes sense that something happened in a past life, which caused a very good birth for me.
Yet something in someone else's past life created a negative birth, perhaps a person born in the middle of Africa, who dies at age 6 from diarrhea. It seems logical that Something happened before my birth to create beneficial conditions. So I don't "know" that Kamma is true, but I believe it because we're not born equal.