In 1963, Arthur Staats published his first treatise on what would be a lifelong mission to develop a unified theory of psychology. His approach was to first argue that the various factions of behaviorism (i.e., Watson, Skinner, Hull, Tolman) shared a core set of assumptions that could be united by recognizing the relationship between emotions and reinforcers. Specifically, Staats argued that all the behavioral positions either implicitly or explicitly connected reinforcement and punishment to pleasure and pain. Moreover, each perspective implicitly or explicitly acknowledged that pleasure and pain were evolutionary mechanisms designed to foster approach and avoidance behaviors. With this lens, Staats claimed we could fundamentally understand how experience builds basic behavioral repertoires, and these repertoires form the building blocks of all complex animal behavior. Staats further argued that by including the construct of emotion, behavioral theories could connect to more traditional (human & mentalistic) psychological approaches that attempted to explain intrapsychic processes. As such, Staats thought his framework could mend another great divide in the field, the one between behavioral approaches and traditional psychology. This is why Staats ultimately came to call his approach psychological behaviorism.
Despite an impressive and broad network of connections, and a research methodology that yielded fascinating results across a wide variety of phenomena, as Staats himself acknowledged, his approach to unification failed. The behaviorists continued to compete about conceptual issues, and complained that Staats did not fundamentally resolve their disputes. And traditional (intrapsychic-mentalistic) psychologists basically dismissed psychological behaviorism as a fundamentally behavioral perspective that could not effectively incorporate either cognition or consciousness. Thus, he ended up pleasing virtually no one (at least relative to the scope of his vision), and, consequently, the only people who are likely to be familiar with Arthur Staats are historians of psychological theory.
Although Staats made a noble effort, from a natural science perspective (and Staats saw psychology as a natural science), his approach was doomed from the start. This is because a unified theory of psychology is essentially impossible. Scientists, especially real scientists like physicists, are a skeptical bunch. If one is going to make an incredible claim like discovering a unified theory that upends current understanding and can serve as the foundation for further growth, one had better generate some incredible and precise predictions, and demonstrate the accuracy of those predictions with experiment. Consider, for example, Einstein's work on general relativity, and the predictions it made regarding the perihelion motion of Mercury (explained by how the sun bends light because its gravity bends the spacetime field around it). These were incredibly precise predictions that could not be explained by standard (Newtonian) models of gravity. This is the way that real sciences (i.e., physics) advance. By generating theories that derive precise predictions which are then measured, and further followed up by experiments designed to rule out alternative explanations. It is this process that beats back the intense skepticism of scientists. Without it, one can never hope to coral the field at a large scale level.
If this is the frame for what real science is about, we can see immediately that psychology cannot be unified. Psychology does not really even have the first building blocks of a coherent science, which is a workable conceptual-semantic system that the practitioners of that science can agree on. Staats' argument was really at a conceptual-semantic level, not at a precise, experimental predictive level. Of course, "a working broad conceptual framework" does not sound nearly as sexy as a unified theory.
There are additional reasons why no one should be venturing forth with a unified theory of psychology. Again, the only possible way a unified theory could be achieved would be via its connection to the hard sciences. Social scientists, humanists, and postmodernists are all generally grounded in a relativistic frame that would make them scoff at the idea of a unified theory. Concerned about the power implications associated with orthodoxy, hierarchy, and foundationalism, these thinkers will raise serious epistemological and moral concerns about achieving a centralized conceptual coherence of psychology, embracing its pluralism as a testament to the human freedom to believe in what defines us. Thus, one will never win a political argument for conceptual unification. So one's only hope to unification lies in the precise, mathematical prediction of animal/human behavior, grounded in the biological and physical sciences.
And yet we know that such foundational scientific grounding is impossible because the bedrock of such a construction, grounded as it must be in physics, is itself not unified. There is NO unified theory of physics. Physicists thought that Newton had shown the light on all the physical world, and prior to the turn of the 20th century, there was a sense that the material world had been effectively mapped. But cracks appeared and by 1920, Newton's ideas had been frayed at the edges of the very small by quantum mechanics and the very large by general relativity. The great Einstein spent the rest of his career, literally until his death in 1955, searching for a unified field theory that would connect general relativity and quantum mechanics. He failed in that quest, and it remains unsolved. Physics is not unified. And indeed, with mysteries surrounding the Higgs Boson, multiverses, wormholes, and now perhaps even violations of one of the universe's foundational constants, the speed of light, any claims that it will be unified seem far off.
In short, conceptual unification will never be achieved as there are too many political forces that pull it apart. Foundational unification could only come via precise experimental prediction grounded in physics. Yet, since physics itself is fragmented, such a dream seems misguided. Perhaps that is why since Staats there really has only been one individual who has devoted his career to tilting at such windmills.
(Note that this is the first in a series of three posts on the unified theory. In the second and third posts, I defend the unified theory from the criticisms leveled here. As I tell my students and clients, it is important to be able to step outside your frame of reference and empathize with your critics.)