It depends on what you mean by "humans,: and what you mean by "meant"

If by "meant" you mean, "what works best" or more precisely, "what actions, when taken by a majority of individuals, provide the greatest chance for the continuing success and survival of the species" then this is an interesting question - but you have to be careful because another possible interpretation of "meant" is, "what is god's plan"

By "humans" you might mean, "modern humans" as in, "what works best for us today in 2008" but keep in mind, our species is about 1 million years old and things changed drastically about 10,000 years ago when we invented agriculture. So you might be asking, "what works best for modern humans" or you might be asking, "what works best for humans in general - of which, the last 10k years is just a blip"

What I'm getting at here is, we are modern humans living sedentary lifestyles in an urban environment and a stratified society. What works best in this environment is likely very different than what works best in the environment for which we evolved - small family groups, living as hunter/gatherers.

As a quick example, Dawkins suggests in the God Delusion that love evolved to bond a pair together for about three years, long enough to ween a child. That works great for hunter/gatherers. But in our modern world, I would argue that it takes nearly two decades to really ween a child, because that's how long it takes to fully educate him and then kick him out of the house.

So if you're asking, "what works best for humans" then I'll answer by telling you what works best in a little rift valley in Africa, and I'll ignore the little blip in time since the last ice age. When you look at the big picture, the last 10k years are insignificant.

But if you're asking, "what works best today" then I'll answer by telling you what works in an urbanized culture. And this answer will be pretty much the exact opposite of the other answer.