Q. My 4-year-old is very aggressive toward his 30-month-old brother. I don't think he's completely to blame for the conflicts, but when he gets mad he sometimes lashes out with a punch or a kick. A therapist friend of mine told me that ignoring their conflict will, as he put it, "facilitate extinction." He also said that punishing my 4-year-old will only make matters worse. (By the way, we don't spank our children and we don't allow them to watch television violence, so I don't know where this came from.) Is there something I can do besides pretending this isn't happening?

A. In a graduate course in experimental psychology, I was given a rat I named "Mad Dog." The assignment was to design an experiment that would demonstrate a behavioral principle, so I trained Mad Dog to turn around in circles to obtain a treat. Then, I "extinguished" his "circle-turning behavior" by stopping the treats. In effect, I ignored him. Mad Dog ran in circles until he was exhausted. Then he started biting me whenever I picked him up, but the circle-turning stopped.

Humans are not so simple. In the first place, ignoring human-on-human violence doesn't "facilitate extinction." It enables it. The fact that you don't "reward" your 4-year-old's attacks on his younger brother by paying attention to them is more than outweighed by the fact that his attacks pay off in other ways. He gets the toy, his brother gets out of his way, he feels the addictive surge of dominance, etc.

You need to put a stop to this, and fast! I recommend punishing both equally if one took violent action against the other. But you're not describing sibling conflict or rivalry; you're describing sibling abuse. Your toddler needs your protection, and your 4-year-old needs to be stopped.

In my estimation, confirmed by lots of experience, the only way to stop him is to punish him. You might have been able to use nonpunitive methods successfully when the aggression first started, but at this point, attempts to "counsel" the 4-year-old into handling his frustration in civilized fashion are going to fall flat. Not only is punishment the answer, but the punishment in question has to constitute an offer the 4-year-old "can't refuse." In other words, the punishment has to be more powerful than the payoff he's experiencing when he hits.

I advocate a zero-tolerance policy. When the 4-year-old aggresses toward his brother, don't remind or warn. Take him immediately to his room, confine him there for the rest of the day, and put him to bed right after supper. Be firm, but not angry.

When he's stopped crying, go in to him and reassure him of your love, but be clear that you cannot tolerate him hitting his brother. The next day, first thing, he must apologize to his brother and give him a kiss. If he refuses, keep him in his room until he sees the wisdom of at least acting remorseful. Also, keep telling him that the way he can avoid going to his room is to come to you when his brother is upsetting him. Assure him of your help with whatever is making him mad.

This approach provides the younger child, who at this point is unable to defend himself, with adequate "violence insurance." For now, that needs to be your priority.

Family psychologist John Rosemond answers questions at www.rosemond.com.