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What do you mean "which personality I want to get"? You don't choose between five personalities, you find where you are on the scale of different personality traits that are present to some extent in everyone.

And yes, you can affect the outcome if you want. You can affect the results of any test if you want. I can make my IQ test look like I'm very stupid. I can cram for some certification exam and forget all the material the next day. The idea is that if people actually want an accurate reading then they will answer truthfully.

I feel like you're conflating two arguments that are unrelated. The fact that the results of psychometric tests can be misused and manipulated by bad actors is a separate issue to whether or not they are scientifically valid when administered properly and taken in good faith. You've admitting that various tests can provide useful --though not 100% accurate-- information. That's the whole point: To provide useful information, which can be further explored.

Psychologists creating and using a test to evaluate five personality traits does is not them saying that there are only five personality traits or that everyone with a given result is the same. The tests are basic, they are primitive in our understanding of how the mind and personalities work, and they have just been shown to be statistically valid enough to be useful in informing other things.

On that note, I completely agree that psych tests shouldn't be used to determine anything of importance outside of clinical settings. As I said, none of this is anything but a primitive understanding and shouldn't be used to determine career tracks or anything about our place in society. I hear about companies using the MBTI (which I personally loathe) to place people and I cringe so bad.