Please don't put out assumptions like "PM = Psychology + Economics" without an indepth argument about it. I completely disagree with such statements and without arguments I can't even attempt to see it your way.
I apologize for not responding to this sooner. I just missed it.
Project Management is about resource optimization, where for the case we're talking about those resources are people.
Optimizing those resources involves balancing the supply and demand for people's hours, knowledge and skills. More experienced and highly capable people on your team will be in more demand, despite the always limited supply of their hours which increases the cost of those hours to your timeline.
Psychology is the study of human behavior. Because the resources are people, a huge portion of your job in optimizing that timeline is optimizing the people. That means understanding motivations, weaknesses, burn out, focus, etc as well as a number of other factors.
