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Are they changing their personality or are they changing their behaviors ? I would think of the personality as the predisposition to certain behaviors. You can change your behaviors to differ from those you're predisposed to; but it takes more conscious effort to maintain those behaviors than it does to maintain the ones you are predisposed to. And changing those behaviors doesn't count as changing your personality,…
If you condition yourself to more regularly exhibit a certain behavior, how is that different from changing your predisposition towards that behavior? Certainly, new habits can be formed and over time people come to exhibit new behaviors reflexively if they've made a habit out of them. Some habits may be harder to form than others, and one's environment will always have a major impact on which habits they're able to…
While it certainly is possible to form new habits, but as long as they go against your nature the price to pay for this is high: you could cultivate a neurosis or depression by doing so.