Could one argue that parental discipline constitutes mental/emotional abuse in certain cases? At what point does punishment (ignoring physical punishment for this question) become abuse?

That parental discipline may constitute a form of abuse depends entirely on what you accept under the label of "discipline". Consider for example a family in which following some religious practices - like preying before supper, or not eating certain kinds of food - is considered as part of a discipline that children are obliged to follow, and a 10 years old child (that is, someone who is cognitively able to take at least partly autonomous decisions on her moral preferences, even is she still doesn't have reached the institutionally established "age of reason", usually 16, 18 or 21 according to the countries) who refuses to comply. In this case, I would consider a sanction of her behaviour as a form of abuse. Punishing her for not complying to a rule she doesn't want to endorse because she finds it incompatible with her ideas and moral feelings is a form of abuse.

Abusing children means prescribing them a system of rules of disciplines without taking their stance and thinking about what is reasonable to accept now and in their future and what is questionable from their point of view now and in their future. Of course it is very difficult to evaluate what are the cases in which we have the right to act in an authoritative way with our children and cases in which we haven't this right, because given the asymmetry in experience, cognition and moral development between our children and us, very often we must force them to comply with practices and norms whose interest and value they cannot immediately understand.

One may argue that paternalism - that is, restricting the freedoms of dependants in what it is claimed to be their own interest - could serve as an appropriate moral test for parental discipline. But I don't think it suffices. In many cases we have the right to decide on behalf of our children in our own interest: we may for example proscribe some of children's actions or behaviours by appealing to the respect they owe us, that is, to the regard they should have to our way of living. So paternalism doesn't suffice to discriminate between legitimate and illegitimate coercion that we exerce on them.

A more plausible moral test for discipline would be the following: we can legitimately impose children to comply with some rules and practices if we consider our attitude towards the values these practices help to promote and the reasons why we endorse them. If the values these practices help to promote (hygiene, respect, instruction etc...) are values that we share and accept on a reasonable basis and not simply on authority, and furthermore we think we will be able in the future to articulate a reasonable explanation to our grown up children on why we decided to force them in such a way when we had control on their lives, then we can legitimately impose a discipline.

A different question is when punishment for not complying with parental discipline can be considered an abuse because, for example, it is too severe. Here I think we should take a long term perspective and balance the corrective effects of the punishment on education with long term effects of the punishment on the emotional and moral development of the child. For example, public humiliation, a form of punishment that was very common in schools and families, should be avoided, because its long term consequences may be extremely negative, whereas any form of punishment that involves taking the responsibility of an action (like for example learning to admit a fault), can have positive consequences on the way the child will conduct herself in the future. It will be useful also in this case to take the children's stance and try to "simulate" their understanding of the causal path which goes from their action to the punishment.

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