The correct answer is: "Many citizens are choosing to have small families".
The situation in developed countries could be considered optimal for child raising: advanced economies with high income levels, good healthcare systems for mothers and children, proper nourishment and hygiene conditions, etc. But the reality is that birth rates are decreasing in most developed countries.
The cause behind this fact is that potential parents voluntarily chose not to have children or two have at most one or two. For example, something which became new during the past decades is that many women have entered in the labor force and some of them priorize their professional careers over family prospects. Another new factor is that marriages tend to happen at older ages, hence a big proportion of the fertile life of women takes place when they are single and not looking for babies.
