In Psychology it has always been opinions and theories, but, that is changing.

Brain scans can show differences in brains. Abuse & neglect for instance changes the brain.
Scans show that physcopaths are different from a 'normal' person they have little or no activity in the Amygdala;
Shown in research to perform a primary role in the processing of memory, decision-making, and emotional reactions,
wiki.

Take love. Its a chemical reaction, Oxytocin the 'bonding chemical' no oxytocin no love.