Short answers, pulled from the story.
Child abandonment is the practice of relinquishing all interests and claims over a child with no intent of resuming guardianship. In the United States it is classified as a class 4 felony, rising to a class 3 felony for a second offense, though all fifty states and D.C. have safe haven laws allowing anonymous surrender at designated locations without prosecution.
Poverty and weak social welfare systems are the most consistent documented causes. Additional factors include social stigma around single or teenage parents, children born outside of marriage, the sex of the child in cultures with strong gender preferences, political displacement and war, parental incarceration or deportation, and parental mental illness or substance abuse.
Safe haven laws allow parents to surrender a newborn at a hospital, fire station, or police station without facing prosecution or further questioning. They passed in the United States in 1999 and have since been adopted in Canada, Japan, France, and Slovakia. As of 2017, 3,317 babies had been surrendered under these laws in the United States.
China's one-child policy, introduced in 1979, resulted in hundreds of thousands of children being abandoned over more than three decades, the majority of them girls. Non-governmental organizations helped arrange international adoptions, and more than 120,000 Chinese children were adopted abroad as a result.
Operation Babylift was a U.S. government program established during and after the Vietnam War to bring more than 3,300 children to Western countries for adoption. These children included orphaned, abandoned, and mixed-race children born to American fathers and Vietnamese mothers, locally called children of the dust. Non-governmental organizations attempted further international adoptions but were largely ineffective at the scale required.
Foundlings appear in some of the oldest surviving texts. Oedipus was abandoned as an infant in the hills and raised in ignorance of his origins. Moses was placed in a reed basket on the Nile and adopted by the queen of Egypt. Cyrus II of Persia and Romulus and Remus follow similar patterns. In Greek literature, abandoned children are typically identified by birth tokens, as in Euripides's Ion.