That is a very interesting point.
However, consider the following take on that same idea:
In considering human behavior, you have to take into account that the choices we make are governed by conscious decision, at least macroscopically. A person’s choice to sit and act rationally will be highly influenced by the long string of rational decisions that led them to their current position in ‘parallel universe space’. If a particular person has made a long series of rational decisions to sit and act normally, then the probability of them suddenly jumping up and acting irrational will be extremely low. The more rational decisions your particular history reflects, the higher the probability that this behavior will continue. Society even influences these choices and molds the choices to be rational on average as the individual grows in age, decreasing the probability of a sudden irrational act. Society also acts on people en masse, affecting the distribution of these probabilities over time, decreasing the overall distribution of probabilities for irrational acts even further.
The probability of the sudden irrational act, and indeed all possible acts, is still there, just extremely low compared to the probability of the rational act in accordance to the history of individual, and the history of all individuals interacting together. The parallel universe in which you suddenly jump up and shout still exists, but in that universe the probability of any number of people doing that as well has the same overall low distribution. Any one universe would only be witness to a very few events of this type.
For an example, simply read the news. There are always seemingly sane people who suddenly engage in irrational acts. These events just happen at a very low frequency in the particular parallel universe in which you are currently residing and reading these words, and indeed in all universes on average.
There is, of course, a universe in which you reside that reflects a different history and the probability of your irrational acts is much higher than your rational ones. However, that universe would contain the same average spread of individuals and your actions would cause you to be the exception there rather than the norm.
As a matter of fact, now that I have typed this and you have read it… our universes have diverged. In my current universe space, you jumped up suddenly from your chair for no obvious reason while the majority of people remained seated. This point of divergence will surely alter your probability distribution in regards to making rational decisions in the future, while the rest of us have just yet another rational choice reflected in our histories.
That being said, I don’t necessarily believe the many-worlds interpretation of quantum mechanics, and neither do I disbelieve it. It is interesting, intriguing, useful for philosophical discussion, but ultimately impossible to prove or disprove. An argument can always be made from an opposing viewpoint just by widening the view to allow a counterpoint. There is no hypothetical limit to the amount of justifications that can be included in the arguments since it is a completely abstract concept.