Most Americans (estimates vary: around 94%, or maybe 85%) hit their children. This includes spanking, slapping on the hand or leg, pinching, shaking, hitting on the bottom with a belt or paddle and slapping on the face. By one year old, around 50% of parents are hitting their children for discipline; the numbers increase with age and reach a peak at 4-5. The most common form of discipline is spanking (which over 70% of parents use). It's not clear exactly how often most children are spanked (partly because people aren't good at recalling every instance): one author suggests three times a week for toddlers; however there is a lot of variance between families here, with some children hit a few times a year, and others being hit daily.
Boys are more likely to be hit than girls, blacks than whites, and poor children than rich ones. Younger parents, or those who are under stress, are also more likely to hit their children. But because spanking and other forms of physical punishment are so common, it's pretty widespread throughout all groups regardless of gender/race/class. It's also very socially accepted: most Americans approve of spanking.
This information appalls me.
There is a wealth of evidence suggesting that hitting children is an ineffective disciplinary tactic, especially in the long-term. There is also good evidence that hitting children has many negative side effects. It's associated with mental problems, including anxiety, depression, drug and alcohol abuse. It increases aggressive and violent behavior, both in childhood and in adulthood. Not surprisingly, hitting children is not recommended by the American Academy of Pediatrics and most child development experts (it's also illegal in 29 countries).
Since even pro-spanking advocates agree that other methods of discipline work at least as well (and in fact often advocate their use), it seems completely bizarre to me to opt for hitting as a parental strategy. In fact, assuming that your goal is to have a well-adjusted, non-violent child, it is downright stupid.
So why do most parents spank? Sometimes people say it's because they've lost control of themselves. I do understand this (I certainly have wanted to smack R before), but it's also troubling: very few people hit other annoying adults, tempting as that is (I've also wanted to smack B and even strangers ahead of me in line: I hate waiting), so presumably they are in fact able to control their violent impulses when necessary.
For most, though, I think it's for psychological reasons. Since virtually all American adults were hit as small children, to acknowledge that hitting children is wrong and harmful means condemning their own parents. It also means realizing that they themselves may in fact be warped in some way. No one wants to do this. It feels like a betrayal (of your parents, who were probably basically decent people trying their best), and is humiliating (admitting your flaws).
Instead people will argue, "I was spanked/paddled/caned/beaten, and I turned out fine!", which praises their parents and reaffirms their own worth. (It always makes me feel especially sad when this statement is manifestly false, like when the arguer is obviously psychologically damaged: I feel embarrassed for them.)
I am therefore never going to spank R (or punish her in any other physical way). I don't even slap her hand out of the way when she's trying to touch something dangerous (a firm but gentle grasp, and guidance to something else, works just fine). She is still really small, so who knows how it will ultimately turn out: but so far I haven't felt the lack of spanking at all.