Suppose your parents are constantly pressuring you: you must eat more beef and lettuce, and less lamb and carrots. It's for your health. And will make you skinny. But dammit you like lamb with carrots and you're tired of beef. And lettuce tastes like dirt. You'd get annoyed with them and you'd pick up they have no real arguments/reasons behind their crap. Well, you might pick that up. But it's hard. Parents bluff. A strong willed independent person will pick it up and ignore them. But that's rare, especially in young people. Not that most people are docile. Many will be unsure and conflicted. Many will sometimes ignore parents but sometimes think they might know something or have a point.
Parents would have hard time doing this alone. If TV was constantly explaining how good for you carrots are, it'd never work. Parents are thus known to complain incessantly about influences (ie sources of information that might reveal their bluffs and lies). But on a lot of issues, the TV isn't going to help much. There are other sources of information as well. Teachers, friends, books, magazines, internet
Overall a young person gets a lot of pressure on the side of your parents. Random adults he meets for dinner will make comments in support of the same bluffs his own parents made. His own friends will face similar lies from their parents, and also be unsure. And the strong independent friends will seem reckless and not good role models.
So, what he really really needs is not one more person saying that maybe his parents are right about carrots. It is someone encouraging him to make up his own mind
Conventional wisdom is true sometimes. So let's pretend you agree with the conventional wisdom about a particular issue. It doesn't matter very much which one. You still face the issue of how to communicate this while remaining sympathetic. Even if the parents are correct now and then, that doesn't mean you should be on their side. So what can you say?
Here's my suggestion:
Before you can rightly say the same thing his parents said you need to comment about how much you agree with him that they are nasty bastards and he shouldn't listen to them. They lie. Then say if they are right it's only by pure luck. Then add stuff about how he should make his own choices and only take your advice if you are persuasive. Then add stuff about how this is not a matter of life and death and he can always change his mind later and this whole issue really shouldn't be a very big deal. *Then* say you happen to think carrots are bad, and give real reasons. (Only do this if he has not heard your set of reasons before. If he is familiar with them, do not repeat, just refer to them and ask what he thinks is wrong with those reasons)
One flaw with the above is that you can't actually tell many children that their parents are nasty bastards. They rightly don't want to fight with their parents. So if you say that, they may be alienated from you. So a real statement often has the even harder task of simultaneously distancing from the parents and being sympathetic to them.
So one possible approach is to say (it really really depends on the person, and your relationship with him):
I saw you arguing with your parents about food again yesterday. Your parents mean well, but they care about you so much that they are over-zealous and over-protective. They are biased and it effects their judgment. So as much as they are trying to help, if your wellbeing is involved ... Their advice is probably perfectly safe but not necessarily the most rational. There are sometimes other choices that'd be good too. So you shouldn't feel compelled to do everything they say. You know that already. That's why you want to eat lamb and carrots, and be a chess player not a lawyer. And I agree with you about chess: being a lawyer is definitely not for everyone and you should try doing something you like. But I wanted to let you know that I actually avoid eating carrots myself (but lamb I do eat now and then). I have a book with me if you're curious about my reasons. It is about zen philosophy and explains why we shouldn't eat carrots. So if you want you can read it and make up your own mind. It's not too big a deal either way, but I thought you'd like to know there are serious reasons people don't eat carrots.
So note some of the key elements:
- Agrees with parent's conclusion (no carrots) without endorsing parents
- Shows seriousness of thinking child should make his own choices by endorsing him in a different disagreement with his parents
- Not hateful towards parents but also says they may be wrong
- Not trying to pressure child, only trying to genuinely offer helpful information
- Has reasons for position and offers them to child so he can evaluate them himself