When is spanking considered child abuse?

Spanking and corporal punishment are a discipline method in which a parent or school official hits a child’s bottom with a hand, a belt, or a paddle. The intent is to modify the child’s behavior without causing serious injury. In 1977, the Supreme Court ruled in Ingraham v. Wright that in-school corporal punishment does not violate the Eighth Amendment’s prohibition on cruel and unusual punishment. Spanking is legal in all states and corporal punishment is allowed in schools in nineteen states.

When is spanking considered child abuse? What are the criteria for child abuse? Is spanking an effective means of discipline? Do parents have the right to discipline their children as they see fit?
Do spanking and corporal punishment have negative effects? If so what are they?
Why has corporal punishment been abolished in 100 nations of the world? (Taylor, Manganello, Lee, & Rice, 2010)
Does corporal punishment teach children that violence is a way to solve problems, particularly when they see adults using it on children?
In some studies of corporal punishment, there appears to be disparities among particular groups of children being hit two to five times more frequently than others. Is corporal punishment in schools discriminatory and which children are being targeted? (Gregory, 1995)
References:

Gregory, J. F. (1995). The crime of punishment: Racial and gender disparities in the use of corporal punishment in U.S. public schools. The Journal of Negro Education, 64(4), 454–462.

Taylor, C. A., Manganello, J. A., Lee, S. J., & Rice, J. C. (2010). Mothers’ spanking of 3-year-old children and subsequent risk of children’s aggressive behavior. Pediatrics, 125(5), e1057–e1065. doi:10.1542/peds.2009-2678