Fewer marriages leading to fewer births, as reported by the Economist:

"... in the 1950s, fertility started to plummet. Since the 1980s, when the birth rate fell below 1.5 children per woman, Japan has, in effect, had a one-child policy—though, unlike in China, it was self-imposed.

It came as a shock to demographers when the 2005 census showed that the number of deaths exceeded that of births for the first time: the population had started to shrink two years ahead of schedule. The 2010 census results are currently being processed and preliminary results are due in February 2011."

Fewer marriages resulting in fewer children also results in less individuals investing themselves in society and that society's future:

“A growing percentage of the population, both married and never married, without children has no vested interest in society, with hitherto unknown consequences for its self-image and sense of purpose,” [Florian Coulmas of the German Institute for Japanese Studies in Tokyo] writes. And even if policymakers managed to reverse those choices and persuade the Japanese to have many more children, the benefits to the workforce would not be felt for 20 years.