Explain the main reasons for the decline in family size (number of children per couple) and what effects fewer siblings have on a child's development

What will be an ideal response?

In 1960, the average number of children per American woman of childbearing age was 3.1. Currently, it is 2.1 in the United States, 1.9 in the United Kingdom, 1.8 in Australia, 1.7 in Sweden, 1.6 in Canada, 1.4 in Germany, and 1.3 in Italy and Japan.
In addition to more effective birth control, a major reason for this decline is that a family size of one or two children is more compatible with a woman's decision to divide her energies between family and work. The tendency of many couples to delay having children until they are well-established professionally and secure economically also contributes to smaller family size. Furthermore, marital instability plays a role: More couples today get divorced before their childbearing plans are complete. Finally, caring for children and providing them with opportunities is expensive—yet another contributing factor to a smaller family size. According to a conservative estimate, today's new parents in the United States will spend about $280,000 to rear a child from birth to age 18, and many will incur substantial additional expense for higher education.
According to the U.S. National Longitudinal Survey of Youth (NLSY), children's mental test performance did not decline with later birth order—a finding that contradicts the belief that having more children depresses their intellectual ability. At the same time, the larger the family, the lower the scores of all siblings. The researchers also found that the link between family size and children's IQ can be explained by a strong trend for mothers who score lower in intelligence to give birth to more children.
Siblings influence development both directly, through relationships with one another, and indirectly, through the impact of an additional child on parents' behavior. Sibling interactions are unique contexts in which social competence expands. Siblings who are close in age relate to one another on a more equal footing than parents and children. They often engage in joint pretend, talk about feelings, tease, deceive, and—when conflicts arise—call attention to their own wants and needs. The skills acquired through these experiences contribute to understanding of emotions and other mental states, perspective taking, moral maturity, and competence in relating to peers.