Should A Toddler Be Punished For Defiance?
From Q and A in Child Discipline By James Dobson (1976)

Many of the spankings and slaps given to toddlers could and should be avoided. They get in trouble most frequently because of their natural desire to touch, bite, taste, smell, and break everything within their grasp. However, this "reaching out" behavior is not aggressive. It is a valuable technique for learning and should not be discouraged. I have seen parents slap their two-year-old throughout the day for simply investigating his world. This squelching of normal curiosity is not fair to the youngster. It seems foolish to leave an expensive trinket where it will tempt him, and then lash him for taking the bait. If little fat-fingers insists on handling the china cups on the lower shelf, it is much wiser to distract him with something else than to pound him for his persistence. Toddlers cannot resist the offer of a new plaything; they are amazingly easy to interest in less fragile toys, and mother should keep a few alternate goodies available for use when needed.

When, then, should the toddler be subjected to mild punishment? When he openly defies his parents' spoken commands! If he runs the other way when called — if he slams his milk on the floor — if he screams and throws a tantrum at bedtime — if he hits his friends — these are the forms of unacceptable behavior which should be discouraged. Even in these situations, however, all-out spankings are not often required to eliminate the response. A firm thump on the head or a rap on the fingers will convey the same message just as convincingly. Spankings should be reserved for his moments of greatest antagonism.

I feel it is important to stress the point made earlier: the toddler years are critical to the child's future attitude toward authority. He should be patiently taught to obey without being expected to behave like an adult.