The study investigates the evolving relationship between family structure and fertility behavior in Japan, marking a significant departure from conventional demographic research by focusing on the broader family context rather than the individual alone. Drawing on data from the 16th Mainichi Newspaper survey of over 3,000 married women, the report documents a rapid transition from a traditional, patrilocal system of arranged marriages toward a modern regime characterized by nuclear families and independent, "neolocal" residences. A critical driver of this shift is the changing expectation of old-age support.

As the locus of family decision-making shifts from rigid, sex-segregated control toward more egalitarian, joint communication between husbands and wives, the study finds that these modern family structures correlate with lower birth rates, suggesting that as the influence of the extended family wanes and women gain greater autonomy in reproductive choices, the traditional motivations for larger families are being replaced by the socio-economic realities of contemporary Japanese life.