What evidence is there that the nuclear family is some sort of evolutionary trait? If anything it's a cultural/religious thing. I believe what would be our "natural" tendency would be for one male to mate with multiple females and impregnate them as often as possible. There is no evidence that "Traditional family values" are the key to a successful economy. The term "nuclear family" is also something that came right around the time of the second world war, an anyone who has taken American History can tell you that the reason our economy thrived in that time was because of the war effort. We also had an extraordinary amount of optimism because at the time we were almost un-arguably the most powerful country in the world.