By far the most important facet of punishment – and the most difficult one for parents – is consistency. Ideally, a parent should quickly discipline the child every single time he or she misbehaves, but in a restrained, even mild manner. A stern word or two is often enough as long as it’s done carefully and regularly. This approach can initially be more of a strain on the parents than on the child. They’re tempted to overlook or forgive some misdeed, if only because they’re tired or because it may spoil the pleasant time everyone else is having. …
You have to do what’s right for the child, and it really is all about being consistent. They have to grow up knowing what’s appropriate and inappropriate behavior.
Nearly all experts agree that children need and want clear rules, and that being held accountable for obeying the rules is a vital feature of healthy development. But rules are helpful only if children know them and understand them, so the brighter the line, the better.
Still curious? The three aspects of deterrence are (1) celerity, (2) certainty, and (3) severity of punishment. Malcolm Gladwell talks about them here.