The proportion of families in the United States with only-children increased during the Great Depression but fell during the Post–World War II baby boom.
[2][3] From 1980 to 2015, the one-child policy in the People's Republic of China restricted most parents to having only one child, although it was subject to local relaxations and individual circumstances (for instance, when twins were conceived).
The premature death of one parent also contributed to a small percentage of marriages producing just one child until around the mid-20th century, not to mention the then-rare occurrence of divorce.
[8] While many only-children receive a lot of attention and resources for their development, it is not clear that, as a class, they are overindulged or differ significantly from children with siblings.
[failed verification][15] In China, perceived behavioral problems in only-children have been called the Little Emperor Syndrome,[16] and the lack of siblings has been blamed for a number of social ills such as materialism and crime.
It is suggested that confusion arises about the norms of ages and roles and that a similar effect exists in understanding during relationships with other peers and youth, all throughout life.
A 1987 quantitative review[21] of 141 studies on 16 different personality traits failed to support the opinion, held by theorists including Alfred Adler, that only-children are more likely to be maladjusted due to pampering.
[24] Toni Falbo & Denise Polit, in their research of only children, gathered 115 studies to address information and evidence for personality, intelligence, adaptability, and relationships with peers and their parents.
[27] In his book Maybe One,[28] the environmental campaigner Bill McKibben argues in favor of a voluntary one-child policy on the grounds of climate change and overpopulation.
The paper concludes that "those who grew up as only children as a consequence of the [one-child] policy [in China] are found to be less trusting, less trustworthy, less likely to take risks, and less competitive than if they had had siblings.
[30] Furthermore, according to Professor Cameron, it was found that "greater exposure to other children in childhood – for example, frequent interactions with cousins and/or attending childcare – was not a substitute for having siblings".
[31] In his book Born to Rebel, Frank Sulloway provides evidence that birth order influences the development of the "big five personality traits" (also known as the Five Factor Model).