Even the lightest smack carries risks
Those who support “mild smacks” often tell themselves that these are painless taps that do not really constitute violence, although children themselves report differently and smacks are certainly intended to cause some degree of physical pain.
While it is true that the occasional light smack does not necessarily cause lasting harm, there is no such thing as a safe smack. For a start, all smacks carry a small (but avoidable) risk of causing unintended physical injury, for example because an unsteady toddler gets knocked over or a blow misses an intended target of a struggling child’s leg or arm and their head or stomach is hit instead.
Smacking children may also impact on their sexual development. Internet searches and prostitutes’ cards show that there are many adults who seek to spank or be spanked for sexual gratification, though there have been few academic studies in this area. It is clear a sexual interest in smacking is not something people are born with, but rather it derives from childhood experiences – hardly surprising when one considers that smacking involves loved parents hitting their children’s erogenous zones in an atmosphere of heightened passion. The sexual behaviour of consenting adults is, of course, their own business but this particular form of sexuality is one few people would choose of their own volition and could have been entirely avoided if their parents or carers (who could not possibly have foreseen this unfortunate outcome) had not smacked them.
Probably the most fundamental danger of the parental smack, even a light one, is that it tells the child that hitting and violence is a legitimate response to conflict or unwanted behaviour. Surely this is the last message we want to send children? The consequences of programming children to accept the legitimacy of violent punishment can be catastrophic. For example, official reports on domestic violence cite alarmingly high levels in the general population of the acceptability of hitting partners for perceived misbehaviour. It is no coincidence that men often call partner-beating ‘a smacking’ or ‘a slap’ and talk about the woman having been ‘out of order’ or ‘deserving’ their violent treatment. Where have they got the idea that it is acceptable to hit someone as punishment? From their own upbringing.
Research shows multiple negative effects of physical punishment on child development
In 2002 Elizabeth Gershoff conducted a meta-analysis of 88 studies on the effect of “ordinary” corporal punishment, specifically excluding studies on “abuse” (i.e. assaults requiring state intervention)  In 2008 the findings were updated and written up in a report Report on physical punishment in the United States: what research tells us about its effects on children. The meta-analysis showed a strong consensus on physical punishment’s many negative outcomes, including eroded parent-child relationships, weak internalisation of moral standards, increased child aggression, violence in later life and poor mental health. Twelve of the studies examined the relation of physical punishment to mental health problems of children, such as anxiety and depression, and eight examined its relationship to mental health problems in later life; without exception, these 20 studies revealed that physical punishment was associated with an increased probability of mental health problems. Thirteen studies investigated antisocial behaviour: in 12 of the 13 studies physical punishment was found to be associated with a higher probability of delinquent and anti-social behaviour. The same near unanimity (four out of five) was found for studies of the relation between experiencing physical punishment as a child and later adult criminal behaviour.
Since the meta-analysis in 2002 a number of other studies have been published on physical punishment confirming these undesirable outcomes or associations. For example, a US longitudinal study of the disciplinary practices of over 2,573 low-income parents found that “spanking” one-year-olds led to more aggressive behaviour and less sophisticated cognitive development in the next two years, even after factors such as family income and structure, mothers’ race and ethnicity, age, and education and the children’s gender were taken into consideration.
It must of course be recognised that no research into child-rearing can ever be subject to the scientific proof of randomised blind control trials for ethical reasons as well as because child development is dependent on multiple, complex and interrelated factors. One consequence of this is that the very small minority who actively support the use of physical punishment because of political or fundamentalist beliefs will always find some tiny island of research on which to stand amid the ocean of evidence contradicting it.
However there is such a mountain of evidence against physical punishment that its supporters do not now seek to disprove its harm, but rather they argue that it is the “wrong sort” of physical punishment that leads to these negative outcomes and that there is no evidence that “non-abusive smacks” are in any way harmful. “Non-abusive physical punishment” is defined as a non-impulsive, open-handed smack to buttocks or extremities of children aged between two and seven, not delivered as an expression of frustration or anger.
The lethal risk of escalation
While the occasional smack may not harm a child’s development, for some parents (though not, of course, for all parents) there is a proven risk of smacking escalating to severe and frequent physical punishment which clearly is harmful.
Ironically, escalation is a predictable feature of physical punishment because of the only apparently “positive” aspect of physical punishment identified in the Gershoff meta-analysis: that physical punishment could be effective in gaining the child’s immediate compliance (though it should be noted it was not the only sanction which secured this). But, though smacking may stop children misbehaving in the short term, research also found that it fails to make them behave well in the long term, so children must be smacked again, harder, when they repeat the misbehaviour. Advising parents to “avoid smacking unless it’s absolutely necessary” or “only use it as a last resort” may have the perverse result of encouraging its use: young children have a limited ability to control their own behaviour and will inevitably fail to do things that they have been frequently told about. After repeated exhortations parents are liable to conclude that smacking can’t be avoided and the last resort has been reached. Many parents step back from this escalation trap. Unfortunately it often the hard-to-reach parents in difficult circumstances who move from smacks to more serious forms of violence.
 See for example Ian Gibson, 1978, The English Vice: Beating, Sex and Shame in Victorian Britain and After Duckworth. Murray Straus has also recently produced alarming evidence suggesting that corporal punishment in childhood is closely associated with coercive sex in later life – see http://pubpages.unh.edu/~mas2/CP-Empirical.htm
 See, for example, the 2003 White Paper Safety and justice which refers to a survey by Mandy Burton et al, 1998, Young People's Attitudes to Violence, Sex and Relationships Edinburgh: Zero Tolerance Charitable Trust, showing that one in five young men and one in ten young women think that violence towards a partner is acceptable in some situations (for instance, if the woman has slept with someone else).
 E. T. Gershoff (2002), Corporal punishment by parents and associated child behaviors and experiences: A meta-analytic and theoretical review. Psychological Bulletin, 128(4), 539-579
 E. T. Gershoff Report on physical punishment in the United States: what research tells us about its effects on children (2008) Center for Effective Discipline, Columbus, Ohio www.phoenixchildrens.com principles_and_practices-of_effective_discipline.pdf
 Lisa J. Berlin, Jean M. Ispa, Mark A. Fine, Patrick S. Malone, Jeanne Brooks-Gunn, Christy Brady-Smith, Catherine Ayoub, Yu Bai (2009) Correlates and Consequences of Spanking and Verbal Punishment for Low-Income White, African American, and Mexican American Toddlers, Child Development
Volume 80, Issue 5, pages 1403–1420, September/October 2009
 Dr J S Lyons, R Anderson and Dr D B Larson, The Use and Effects of Physical Punishment in the Home: A Systematic Review, presentation to the Section on Bio-Ethics of the American Academy of Pediatrics at the 1993 Annual Meeting