It’s actually self-evidently true.
Our personalities are a product of 3 forces acting upon us during our life:
a) Our genetic make-up,
b) Our upbringing (parents, caretakers)
c) Our environment (pressures, social network…)
Since our parents/ancestors literally transfer their own genes on us, and then they raise us, 66% of our personality is already up to our parents.
Now, thanks to those 66% we “choose” our own environment, our choice being dictated by values shared with us by our parents/caretakers and physical proximity, so even the environmental pressures and social network are mostly dictated by our parents.
In the end, it seems that 99,99% of what makes us “us” is given to us, forced upon us, and we didn’t really end up having any choice.
Does personality matter so much in our decision making? Yes, it does. If I know your personality I can quite accurately predict your behavior in many contexts.
It’s far from proving the determinism, but it’s a good start.
The only thing which breaks up hard determinism is quantum-ness of our consciousness which causes random oscillations in our personality coupled with quantum-ness of our environment (physically and through consciousness of others in our environment), but what is randomness, really?
I, for one, don’t believe in randomness, at all. I strongly believe that if we somehow knew all the variables which decide the output of a system, we could predict it with 100% accuracy.
And, I have a book about 4 types of randomness from Stephen Wolphram which speaks in my favor, and a legendary quote by Einstein that does the same:
God does not play dice.