Should seem like a no-brainer. Yet, in the moment, it's a difficult thing to control. How about a scenario?
Chris and Brian are two young brothers about two years apart. Chris is the bossier, rambunctious older brother, and Brian, his quiet, gentle counterpart. They play well together, but sometimes Chris is a little too rough when he plays. Chris begins grabbing at Brian's face playfully.
"Chris, keep your hands to yourself, bud," you say calmly. Yet, it seems like maybe you are invisible, because he's now hitting Brian's face, and Brian is a little bit in shock. And so are you.
"Chris! Stop that!" He stops and looks at you, and you give him a time-out. But he begins to whine: "Nooooo.... I don't want a time-out!"
"Get going" you say, and maybe stand up and get closer in case you need to guide him. His reaction is to whine louder, almost shouting, "No! I don't want to!" and kicking his feet in your direction. You give him some space, try bribing him with special things he can earn afterward, give him a time limit to get there, etc. Nothing works, he acts like he's going to go, but as soon as you turn your head, he's inching his way out of time-out.
Now you mean business. You grab Chris and make him look you in the face and say "Get in that corner right now, or so help me...!" His eyes widen with fear and he begins to cry uncontrollably, but is submissive.
Did yelling work? It depends on what your goal was. If your goal was to win by any means necessary so he would learn his lesson, then yes, yelling worked. For now. The problem is with the goal, not the method.
Children are born into the world, having no concept of morals, roles, identity, etc. until another human shows them what it means to be human. It is the adult's responsibility to TEACH the child the ways in which he or she should go, and how to behave. Is it the child's fault they don't understand or haven't learned to listen the first time you say something? No, the child needs to be taught how.
1. He won't listen to you unless you're yelling.
In making your goal short-sighted, to "win" the battle with your child, you also short-change him. While you are busy winning, you are teaching him that he is only doing wrong if you are yelling. As long as you are not yelling, he is doing just fine. You have just trained him not to take you seriously when you are speaking calmly, but only when you are angry and in his face with threats.
2. He lost respect for you.
He has learned not to respect you, because when you are calm, you hold nothing to the power and fear you can instill when you are yelling. Then, when he obeys to a yelled order, it is out of fear, not respect.
3. He will start yelling at you.
By far, the best way children learn is by example. It starts with clapping, saying "Mama" or "Dada", repeating what you do, and what you say. It is the primary mode by which children learn. Since children only know what they have been taught, yelling at your child teaches them that calm communication fails: you must yell and threaten to see results. This will carry on as truth until he is shown another way.
4. His behaviors won't change.
But they did, you think, he went to time-out. He will continue to hit his brother, he will continue to blow off your first prompts. Again, because you trained him not to listen to you when you're calm. If you say "Stop hitting your brother" and "Go to time out" with a calm voice, those will be ignored, because he feels he is not really in trouble unless he is being yelled at. That is the limit that has been set by you, so that is the limit he will go to.
If you have to resort to yelling almost every day with your child, then it is obviously NOT WORKING. A great quote I try to remember is, "If you always do what you've always done, then you'll always get what you've always gotten" (Anthony Robbins). Yelling more won't fix it, either. It's time to give yourself and your voice a break! You don't have to yell to get your children to follow your directions; it only makes things worse. For what to do instead, I suggest "Parenting with Love & Logic" by Foster Cline and Jim Fay as an effective approach to gaining compliance with your child. I think I can share some of the basics, but that's another post for another day.