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.