I have heard many claims that spanking is ineffective for disciplining kids? Is there any research to confirm or deny this?
There is evidence that corporal punishment (CP) increases the risk of future agressive behaviour of the child ("Mothers' spanking of 3-year-old children and subsequent risk of children's aggressive behavior" published in Pediatrics). They state the following results of their study
Frequent use of CP (ie, mother's use of spanking more than twice in the previous month) when the child was 3 years of age was associated with increased risk for higher levels of child aggression when the child was 5 years of age (adjusted odds ratio: 1.49 [95% confidence interval: 1.2-1.8]; P < .0001), even with controlling for the child's level of aggression at age 3 and the aforementioned potential confounding factors and key demographic features.
From another study titled "Parental corporal punishment predicts behavior problems in early childhood" published in the Journal of Family Psychology
The results indicate that parental CP uniquely contributes to negative behavioral adjustment in children at both 36 months and at 1st grade, with the effects at the earlier age more pronounced in children with difficult temperaments.
A review titled "The risks and alternatives to physical punishment use with children" published in the Journal of Pediatric Healthcare states in the abstract
Despite strong evidence of negative developmental outcomes resulting from the use of physical (or corporal) punishment with children, its use by parents and other caregivers is common. Such negative outcomes include child aggression, mental health issues, and physical abuse.
The research on very mild forms of corporal punishment is not conclusive, as Dr. Kadzin states in an interview:
Research on very mild, infrequent spanking (e.g., one time/month) is inconclusive. When a parent moves beyond that to moderate or severe physical punishment, there are all sorts of untoward consequences—educational delays, psychological disorders and physical disorders, too.
An article in Scientific American summarizes the results of an APA task force that reviewed the literature on corporal punishment
In a presentation, she explained that the group of 15 experts in child development and psychology found correlations between physical punishment and an increase in childhood anxiety and depression, an increase in behavioral problems, including aggression, and impaired cognitive development—even when the child’s prepunishment behavior and development were taken into consideration.
One out of the 15 experts on the task force disagreed with the conclusion and recommends spanking as a backup when other way of disciplining don't work
There seems to be a general consensus that corporal punishment can have a negative effect on the development of a child. As a consequence, parents should choose methods to discipline their children that have fewer negative effects.