There is a broad body of social and legal research that shows marriage is the best structure for the successful raising of children. A child that grows up out of wedlock has a greater chance of experiencing problems in psychological development, health, school performance, even in the quality of future relationships. Marriage may not be the perfect solution for everyone. But when it comes to raising children, it certainly is the least imperfect of all available family structures.
Has it been shown that this is due solely to marriage, or due to the type of relationship that tends to lead to marriage (in North America, given that I suspect that these studies were mostly taken there)?
If you have two people in a horribly dysfunctional relationship, having them get married is not going to make their relationship any better, and if you have two people in a loving, monogamous relationship, the fact that they're not married does not make their relationship any worse. It seems to me that marriage would be more an
effect of being in a loving, monogamous relationship rather than a
cause of such a thing.