I am asked on occasion why it is so bad to spank and my usual answer includes that there is always a better alternative, one that actually addresses the specific issue and that I don't want to teach that if you are bigger you can be in control because of that fact. I came across a really thorough article about all the actual scientific arguments. Why You Shouldn't Hit It is pretty long, here are a some excerpts, but I really recommend reading the whole thing.
What do scientific studies prove about corporal punishment?
"The negative effects on children include increased aggression and noncompliance—the very misbehaviors that most often inspire parents to hit in the first place—as well as poor academic achievement, poor quality of parent-child relationships, and increased risk of a mental-health problem (depression or anxiety, for instance). High levels of corporal punishment are also associated with problems that crop up later in life, including diminished ability to control one's impulses and poor physical-health outcomes (cancer, heart disease, chronic respiratory disease). Plus, there's the effect of increasing parents' aggression, and don't forget the consistent finding that physical punishment is a weak strategy for permanently changing behavior."
Why do parents still do it?
"The key is corporal punishment's temporary effectiveness in stopping a behavior... The direct experience of that momentary pause in misbehavior has a powerful effect, conditioning the parent to hit again next time to achieve that jolt of fleeting success ... The research consistently shows that the unwanted behavior will return at the same rate as before. But parents believe that corporal punishment works, and they are further encouraged in that belief by feeling that they have a right and even a duty to punish as harshly as necessary."
So what's the case for not hitting?
"It can be argued from the science: Physical discipline doesn't work over the long run, it has bad side effects, and mild punishment often becomes more severe over time. Opponents of corporal punishment also advance moral and legal arguments. If you hit another adult you can be arrested and sued, after all, so shouldn't our smallest, weakest citizens have a right to equal or even more-than-equal protection under the law? In this country, if you do the same thing to your dog that you do to your child, you're more likely to get in trouble for mistreating the dog."