For example, if Japanese households held on to the single-male-wage-earner model for longer than US households then the aging of the Japanese workforce would not have affected the female Japanese labor force nearly as much.

FORBES: Japan, Demographics and National Productivity

The variable Stock and Watson ultimately cite is the decline in labor force participation, and they argue it is driven by the aging of the workforce and the overall distribution of workers by age.

FORBES: Does An Aging Population Hurt The Economy?