The setup for the idea is this: Take two people who are in a relationship and pick a single personality trait or attribute. Each person probably has a specific comfort zone with respect to that trait. That is, they have an equilibrium state (or narrow range of states) outside of which they feel an imbalance and a desire to return to "safety." I'm sure we can all relate to this on an individual level. Now, since we have two unique individuals here, I think it's safe to assume that their comfort zones are different for some set of traits (probably many, depending on how finely you want to slice the notions of traits and significant differences).
In this situation, my theory says that each person will seek the same equilibrium state for the relationship as a whole as they would desire for themselves individually. If one partner creates an imbalance for the other (as may happen naturally if their comfort zones are different) then the other partner will try to counter that imbalance by behaving in the opposite direction, even to the extent of leaving their own comfort zone in that direction. The drive is for the relationship average to be at the desired level, though the complication is that each person may have a different goal.
For a specific example, let's assume one person (A) is an introvert and their partner (B) is an extrovert. If A feels like they're going to too many parties, then s/he may start lobbying for more time at home, or even start deliberately avoiding social events that s/he might otherwise have enjoyed. B might in turn react to this reaction, and be more insistent about going out on principle, rather than for any such intense desire. You can construct many other similar examples, such as one partner becoming more and more effusive in displays of emotion because they feel their partner is distant or unemotional. (And that partner then withdrawing further if it makes them uncomfortable.)
If you want to see this more formulaically, let's take our two people A and B, call the trait T, and the respective comfort points for this trait Z(A,T) and Z(B,T). Then the theory is basically a function that says:
if Z(A,T) < Z(B,T)
then Z(B,T) += Z(B,T) - Z(A,T)
and vice versa
This is, of course, only a rough approximation. Clearly it couldn't strictly be true, because the positive feedback loop it creates would turn each person into a raving maniac at opposite ends of any given spectrum within minutes. So I'd give it at least the following caveats (there may be more):
- This applies mainly to traits or issues of relative significance to the relationship. (E.g. your favorite color doesn't count.)
- Each person probably has a limit to how far out of their comfort zone they're willing/able to go, even voluntarily.
- Some people may be more susceptible or sensitive to this than others.
- This probably applies differently at different stages of a relationship. (E.g. not so much in the beginning, when you're all love-crazy.)
But it's the general concept that's most important here, and the potential application of it. The problem with these attempts to maintain balance is that a mathematically balanced relationship is still very unstable if one or both of the individual partners are working at their limits. However, if you're aware of this potential motivation behind your (and your partner's) behavior, then you can take steps to balance things in a more healthy way.
So how do we do that? Unfortunately, I haven't had much direct experience in this part of the theory yet. My instinctual advice would be to start with that all-purpose relationship tool, communication. If you notice this happening, say so. Explain where your comfort zone is, and describe the things that make you uncomfortable and why (being respectful, of course, of the other person's valid choices and preferences). Maybe you'll be able to find the overlap in your zones and then consciously start spending more time there, but maybe it will be more complicated than that. The greater the differences, the more compromises will need to be made.
The Ideal Balance
This last bit may come as a surprise because I do not believe that the ideal balance has two partners lining up conveniently in every way. Rather, I think this theory illuminates something I've wondered about for a long time. That is the tension between two theories of attraction: the "birds of a feather" model vs. "opposites attract." I've observed both of these effects within the same relationship, without fully understanding how they can both contribute to making things work. Now I think I have a better idea.
The "birds of a feather" traits, I believe, should be the ones that correspond to strengths for both people. If you feel that you are already at the best possible state with respect to one trait, then a good match for you will be there as well. You don't want someone who will be pulling you off track. For instance, someone who is very good at expressing love and caring for people would do better with another similar spirit than with an insensitive brute (to take an extreme example).
The "opposites attract" factor will best come into play with those traits that you feel need some work. (I'm assuming here that you have the desire for self-improvement here, and aren't just apathetic about it.) In this case, an "opposite" partner can help pull you in the direction you need/want to go. For instance, the introverts can get more good social interaction by tagging along with their extrovert partners, who can then in turn perhaps learn something about introspection and self-reflection. Of course, in these cases you have to be very careful. If a growth opportunity for one person is a frustrating drag on the other, then there's still going to be difficulty. And even if there is an ideal midpoint for both people, it may take some compromising to get there.
So I might sum this up as: Reinforce your strengths and balance out your weaknesses. Sounds kinda obvious once you get past all the verbose explanation of it, but it can be harder to see when you're actually in the midst of figuring out a relationship. What do you think?