This is the "income effect". 13 In his well- known work Becker (1981) argued however that if the increase in household income were due to an increase in wage rates (., an increase in labour productivity), the cost of children would increase, because time is involved in producing and rearing them. But other things being the same, this would lead to a decrease in the demand for children (this is the "substitution effect"). It follows that a rise in income owing to an increase in labour productivity would lead to a decline in fertility if the substitution effect were to dominate the income effect, a likely possibility
