It's time for another generalization, and here it goes. First-borns tend to hang out with first-borns, and later-borns tend to hang out with later-borns. (First-borns meaning the eldest sibling in a family or the only child, and later-borns meaning siblings born later.)
If you're skeptical, just have a look at the social groups you're closer to not merely by virtue of organization, but by virtue of how genuinely well-connected and on good terms you are with them.
Research has found that first-borns tend to be less open (more conforming, traditional and closely identified with parents), more conscientious (more responsible, achievement-oriented, serious, and organized), more antagonistic (less agreeable, approachable, popular and easygoing), and more neurotic (less well-adjusted, more anxious). They are also more assertive and leaderly.
What accounts for the difference? Evolutionary psychology theory proposes that each child develops in a different family ecology, forms a different strategy for surviving childhood and attempts to establish his or her own character given the pressures of living in resource-limited conditions, which are partly developmental and partly political where siblings compete for the affection and attention of parents.
First-borns attain the attention of parents by virtue of simply being alive first, and therefore see newcomers as usurpers to their thrones of existence. First-borns thus identify more with parents and should resist changes to the status quo, which has served them well so far, making them conservatives and bullies. Second-born children have to cope in a world that has this obvious obstruction to the attention of parents, and thus would cultivate opposite strategies, becoming appeasers and cooperators. With less at stake at the status quo game, they should thus be receptive to change too.
In this sense, it might appear that first-borns tend to be more independent, while later-borns tend to be more community-oriented.
Interestingly, it was found that in all sorts of shake-ups (such as the Copernican revolution, Darwinism, the French National Convention, the Protestant Reformation and the American reform movement), later-borns were more likely to lend support, while first-borns were more likely to be reactionary and opposed. Later-born scientists are also less specialized, indicating that they are more willing to try their hand in a greater number of scientific fields.
With such fundamental differences (not in terms of hobbies and interests but in terms of principles, outlooks in life and moral values), it is no surprise if it is the case that first-borns tend to associate more naturally with first-borns.
If you find yourself an exception to the case presented above, it is likely that the effect is mitigated by gender and age-gap. Say, for example, that you are a first-born who has more later-born friends. It is likely that your later-born friends have elder siblings of opposite gender, and/or their elder siblings are considerably much older, so they actually behave in a manner that is less affected by the presence of other siblings (and thus appear to be more first-born-like).
The effect of one's birth order in the family on one's character is particularly due to the nature of competition. By that logic, then, one needs to compete with a sibling more if the sibling is of the same gender (where there are more similarly relevant things to compete about) or if the sibling is close in age.