That is to say, we have been thinking about how people think and why they behave the way they do for a very long time, but we only recently began writing down predictions and testing them. This is when the rapid development of the field exploded and is another testament to the benefit of the scientific method, when used appropriately. This is what happens when you make a prediction, which you have been carefully cultivating over decades of painstaking research in collaboration with your colleagues, who have become some of your closest and only friends: all of those friends and colleagues spend the rest of their careers trying to prove you wrong. The history of psychological development is littered with dalliances and schisms, and would make for a terrific, if geektacular, soap opera. This theoretical cannibalism has a similar effect to the purification of gold, in that this trial by fire burns away the parts of the theories that don't work. This does not prove the theory. This is a common misconception. Science doesn't really prove anything. Good science disproves things. You could consider science as an ongoing voyage of discovery of all of the ways that we used to be wrong.
Hall, Freud, and Piaget wrote some predictions down and a whole bunch of people tried to prove them wrong, and managed to do so in some areas and failed to do so in others. This is how the subsequent fields of developmental psychology grew, out of trying to explain the things that Freud and Piaget did not explain. A large part of the language of this dialectic was informed by some concepts that Hall, Freud, and Piaget used, namely that children go through a process of becoming adults, early life events influence the trajectory or nature of this process, and that the process appears to go in a progression of steps, which they called stages. As a result, most of the theories proceed in stages, though this is mostly a tool to note particular events in the process in relation to each other chronologically.
Freud was singularly important to the general field of developmental theory, as the first person who really connected early life events to later life habits and dysfunctions. This seems like the kind of thing that can come across as being obvious, now, but that is because all of us got to grow up in a world where Freud had already demonstrated this reality. At the time, this was a revolutionary concept, and the rest of the field spilled out of it.
There seems to be a feeling that the disparate psychological theories can't be synthesized. Usually, this conversation wraps around thrusting one theory against another and claiming some kind of incompatibility. You might hear conversations like this, maybe around nature versus nurture, or attachment theorists decrying self-regulationists. On occasion these conversations can become heated and might seem to be approaching what you could call an argument. I have, myself, been a part of some conversations like that. I would suggest that most of these style of conversations fail to take into account the complexity of human beings, and want to insist on one answer that explains everything. In rationalist circles they call this a false dilemma. The rest of us understand it a little more clearly as the superhero dilemma. You know the classic scenario where the monologuing villain dangles the bus full of school children from one hand and the hero, or heroine's, love from the other, declaring that the hero must choose! One is doomed to die! Insert sinister laugh track.
What happens next? Against all odds, the hero manages to find a way to save them both. This is a great example of humans recognizing in their hero the very skills they need to be successful in life. The real world, and the people in it, are not binary. You don't face either-or decisions. There is an entire universe worth of possible solutions to any problem, and framing your problems in an either-or set does nothing but trap you into selecting between two, and usually not the best two, of your nearly infinite options. The same trap can happen in theoretical psychology. Freudians and Jungians don't talk to each other at parties, and Wilberians don't even get invited to those sort of parties. I want to suggest that a binary presentation of options never results in the best choice possible being made more likely.
To the point, people are complicated. This is probably not a surprise to anyone here, as all of you have met people. Some of you even are people. You know that people are complicated. In fact, the wide variety of psychological fields is itself a testament to the variability and complexity of the human species. Or at least of the human individual. I think that also applies to the species, but that is a correlation that we have not made, yet.
Psychology has a lot to say about the ways people are different. Entire fields of psychology exist just to help us talk about the precise nature of those differences in greater detail. Piaget, Erikson, Maslow, Kohlberg, Gilligan, Loevinger, Baldwin, and Freud (and others, but these serve to illustrate the point), DO NOT NEED DISAGREE about how human beings develop. It can be true that I have different ways of interacting with the world, and that each of those interaction types needs to be informed and molded. I have a very powerful biological urge to procreation that I need to figure out how to understand and healthily apply in a complicated world. I also have real emotional sensitivity to the world and to people around me, and I have a need to figure out how to be aware of that and process what is happening. I also have the ability to think about things and solve problems. I can also work with other people. Amidst all of that I have a fluid impression of myself and my place in the world. All of these things are true about everyone. The reason things get so messy, is that we have a difficult time sorting out all of the different ways that we can understand the world. Things get twisted.
So all aspects of being a human go through a gradual process of becoming more aware of what they do, and, hopefully, thereby becoming more effective at what they do. Understanding how thinking works can make you better at thinking. At least, that's the goal. This is where things get even more complicated. At each stage, each aspect has to deal with a particular developmental challenge. If a particular aspect fails to deal with its particular challenge, the ENTIRE INDIVIDUAL develops a particular dysfunction. This dysfunction then makes it more difficult for almost every other aspect of the individual to navigate their own developmental challenges. Each aspect of an individual has to navigate EACH DEVELOPMENTAL STAGE in order for a fully functional, healthy adult to come out on the other end.
The multiple aspects of an individual and the different stages of development provides about 5,040 possible places in the human developmental process that an individual can occupy at a particular moment. Which is not that bad, really, when you consider that there are 7 billion of us, but this is complicated further by personality differences. There are, generally speaking, around 56 different combinations of Jungian personality types, as just one example of a description of the different ways people represent themselves. Personality types are ONLY the way we EXPRESS our mood, reaction, thought process, etc... Personality types, alone, do not tell us how those reactions happen. They just gives us a language with which to interpret them. Think of personality types as a window. There are lots of different windows that can change the way that you see what is on the other side of the window. There are stained glass windows, prismatic windows, two-feet-thick windows, water filled windows, and reflective windows, all of which change how you perceive what is behind the window, but none change what is on the other side of the window. What we have is 56 different windows through which we can see 5,040 possible realities.
Now we just throw these numbers into the mathematical formula for variable possibilities, Newton's Binomial Coefficient. As there is no consensus on number of aspects or on number of stages, I will take the low numbers. Let us say that there are only 7 relevant or critical aspects of human development that only go through 6 developmental stages, and 8 personality types of which 2 are predominantly active. Our equations to represent that would be:
7!/1! Which is just 7!, or 5040
8!/(8-2)!, 8!/6!= 8*7= 56
5040! = 4.529260254362 E16473
4984! = 2.838625656938 E16266
What we end up with is a big number. This number represents, roughly, how varied humanity can appear. If we recall our quantum mechanics from earlier, we could consider this a representation of our Strangeness. In all those possible numbers there is one, and exactly one that represents you. That number is your Strangeness. And each of those other numbers represents the possibility of someone else. We have a difficult time grasping that. Actually, it is impossible for us to understand that in a way that makes an emotionally actionable difference. This, by the way, is where we turn another corner. So far, psychology has just told us that we are so staggeringly different that IT IS LITERALLY IMPOSSIBLE FOR US TO APPRECIATE HOW DIFFERENT WE ARE. If this was where the story ended then we should all cry ourselves to sleep and resign ourselves to a life of conflict, misunderstanding, pain and misery, probably alone. But this is NOT where the story ends, because psychology also tells us why we have such a difficult time with large numbers.