According to some experts, why do birth rates fall when a country’s economy’s improves?

is this question from a book? ?

nationalization is the set of dispositions and operations by means of which the state assumes, in varied form, the administration of private companies, of groups of companies or of the totality of certain economic sectors previously managed by individuals, such as the land portions. nationalization is the opposite of privatization. it is also called nationalization of the means of production, although a nationalized company can be previously both national and foreign ownership. nationalization or nationalization is a very characteristic action of ideologies such as socialism and nationalism.