WHY do couples who live together before marriage have a higher separation/divorce rate?

I'm not sure, but I'll speculate. If you live together before marriage, you probably don't value the covenant of marriage as much (if you did, you would get married). While living together, there is no covenant made before a judge or a priest, there are no witnesses, there are no vows offered, no formal wedding. You owe nothing to your partner and they owe nothing to you. If you dislike the relationship, you can leave at any time. When you decide, finally, to officially tie the knot...well, all that has changed is you now have a piece of paper stating you are married. However, if you honor and respect the covenant of marriage from the beginning, and you decide to wait until marriage before cohabitating, then I think both people are more fully invested in the promises made to each other, before the state, and before God. Therefore, I believe these people will be less inclined to give up on their marriage later on in life. Who knows really. Just my opinion.  more

Assuming this is true (I think the data may be inconclusive), I would tend to agree with Easthill's answer (phrased slightly differently). It is likely the case that people who are willing to co-habitate prior to marriage are also more open to the idea of ending a marriage if things go awry. Conversely, those that have strong religious/moral beliefs about only 'shacking up' within the confines of marriage also tend to be those who take the "until death do us part" more seriously (no matter how miserable they, their spouse, and any offspring are...:-P...).  more