The two modern giants with regards to the study of motivation are Edward Deci (pictured on the right) and Richard Ryan (on the left).
Both Deci and Ryan are Professors (at the University of Rochester, USA), psychologists and authors.
I first came across their names when noting (what I consider to be) the core evidential references used within Alfie Kohn’s well-known book ‘Punished by Rewards’.
Deci began his studies at Hamilton college – the same college that the renowned behaviourist B.F. Skinner had attended around 40 years earlier. As such, Deci was taught the principles of behaviourism and reinforcement. However, Deci’s subsequent PhD research challenged the weaknesses within behaviourism and led him on his long journey of interest into intrinsic motivation.
Deci and Ryan began collaborating with each other on the subject of motivation (I think back in the early 1980’s). They released a 1985 paper together titled ‘Intrinsic motivation and self-determination in human behaviour’.
This went on to be formulated into the well-respected Self-determination theory (SDT) which is a theory of human motivation, personality development, and well-being.
Deci and Ryan have continued their collaboration, and many other psychologists have adopted, extended and expanded their work worldwide.
SDT is currently comprised of a set of six ‘mini-theories’, which you can read about here
I’ve observed that there are a few (expensive) books out there on self-determination theory.
However, if you don’t want to spend the money or get bogged down in the detail and, instead, want to read a relatively short but (in my opinion) highly interesting book that sets out what motivates us (and why) then I’d recommend Deci’s 1995 book titled ‘Why we do what we do’.
You can learn more about self-determination theory at selfdeterminationtheory.org