One thing this living arrangement does is further suppress the birth rate; a rate now at 2.05 births per woman in the U.S. as compared with 1.38 in Italy. Low birth rates create situations in which the taxes of few adult workers support many retirees, the situation now in Japan and most of Europe.
It could certainly be argued that a low birth rate is an unintended consequence of national development. Evidence points in that direction; see this Wikipedia website listing "sovereign states and dependent territories by fertility rate."
The U.S. birth rate remains relatively large for a developed nation because of our high level of (mostly illegal) immigration from undeveloped, high fertility nations.
