The thought occurs that whether one is an introvert or an extrovert by nature could well be determined by genetic factors. Reflection shows that in the animal world there are basically three types of behavior. Those would be herd, pack, and solitary. Considering this and comparing human extrovert/introvert behavior to the herd/pack type animal one sees striking similarities. The third type of behavior, solitary, would seem to be counter-survival to humans and would have likely bred out of the species, so we can probably ignore it in this dissertation.

Individual safety can be considered a dominant need of any species. Therefore, we can see that an individual would feel safest within a large group of similar individuals (the herd) if it had the herd instinct. Similarly, it is evident that an individual would feel safest alone or with a few trusted companions (the pack) if it had the pack instinct. This, I think, exactly matches observable human behavior of the two types of personality.

It is easily to see how having a herd instinct would cause an individual to feel more comfortable with many other herd member around it to protect it. The reason they would naturally be suspicious of anyone who didn't fit in or seemed different is fairly obvious. The herd member does not necessarily need to know the other herd members individually for any animal which looked or acted differently could well be a predator and thus a threat. It would follow that the herd instinct must provide two reciprocal behaviors. These are the individuals need to fit in to act like its fellow herd members, and the herds need to drive out anyone who seems different and thus possibly dangerous. One can see where change that does not involve whole herd would be very uncomfortable. The noise and bustle of the herd is very comforting to the herd members. The individual feels distinct discomfort any time it is separated from the herd. As you can see this is very similar to the behavior of the extrovert.

On the other hand an individual who has the pack instinct would consider any it did not personally know as a possible threat (or as prey because the pack instinct is seen, of course, most often amongst the predators). Here there is a driving need to personally know those around it. Until it is sure that another individual can be trusted and thus be considered a pack member it is going to react by attempting to drive the other away. The pack member is often called upon to act alone, so it is comfortable when it is away from its pack and able to function effectively. To the individual with the pack instinct noise is usually a signal that it should be ready to attack or run. Thus, it and its companions prefer quiet. In fact, noisy environments tend to bring out a quite vicious and aggressive response in them. Here we can see similarities to introverted human behavior.

The herd individual can go into a rage and the other herd members just get out of its way until it calms down. If we view the pack individual as basically a predator we can see that it has to have a much higher level of control otherwise it could hamstring and rip the throat out of any that got in its way before it realized what it was doing.

When mankind lived in tribes, villages and small towns both types of behavior would have been equally functional. The population would be a large enough for an individual with the herd instinct to feel safe; and would at the same time be small enough for an individual with the pack instinct to know everyone and therefore also feel comfortable. Thus through most of human history there would have been no real pressure to select for or against either type. One could even argue that both genes were present and behavior dependent on which was dominant. Now however these differing types of behavior are often a cause of deep misunderstanding and distrust between the two types of personality.