Belief system. We do everything for infants when they cry.This sets up a belief system that we are their servant & only exist to please them.When they begin walking we begin to ignore them, since they can get around & play.They don't like it.Acting out is an amplification of crying behavior.They do it to get noticed & get their way.Parents may find it cute & send a smile their way while saying no-this confuses.
Age and stage? Behaviors serve a purpose, like avoidance of a task or attaining a desired object. Children " win" when they get an adult engaged in a fight; therefore, coercive parenting methods such as yelling, spanking or even becoming upset are a " payoff". State the rule and the consequence, give no more than one warning, then follow through. Walk, don't talk" for successful time-outs, a minute/year of age.