Two major provable differences between linear and non-linear systems are:
Difference #1:
Linear system - The impulse response (see note#1) is
unique for a given linear time invariant (LTI) system;
Non linear system - Multiple systems can have
the same impulse response.
Difference #2:
Linear system: The logical order (ie: the sequence) of LTI subsystems produces
the same impulse response;
Non linear system: The logical order (ie: the sequence) of subsystems produces
different impulse responses.
Note #1: Any LTI system can be characterized entirely by a single function called the system's impulse response
see here.
Human behaviours (when it comes to choosing) could be categorised under these system types but the important aspect is to consider the behaviours across
the whole population of choosing humans .. and not just the ones one wishes (or holds beliefs about).