Personality is people's typical ways of thinking, feeling, and behaving. One research, which is based on Big Five traits model, has shown that personality is not distributed even across the US; the Big Five traits differ across geographical regions. Here is a video to show the results. The Lilienfeld textbook says that culture has influence on the Big Five, which may be one explanation to this phenomenon. The article also says that research data has shown that culture do shape both individual behavior and collective personality traits in a region. However, Personality similarities among people in close geographical proximity may have different explanations.
On one hand, the arrow may reverse. It's possible that birds of a feather flock together. People go and live in places that meet their needs. For example, people who concern safety may prefer to live in small towns, while people who desire extravagant life prefer to live in big cities. Besides, there's also empirical evidence that attitudes, opinions, and emotions are contagious. Perhaps we all have experience of catching negative emotions which are hold by someone around us and becoming depression without specific reasoning.
On the other hand, People from a specific region are more likely to share a single gene pool; geographically proximate cultures often have common ancestor. Therefore, it's possible that not the culture, but the genetic factors lead to personality similarities. Obviously, we go back to the question of nature and nurture--the big topic in psychology.