The purpose of this research paper is to find a correlation between peer, genetic, and environmental influences and the behavior and personality development during childhood. This study focuses on the 1999 Columbine High School Massacre to try to explain certain kinds of behavior. The study made on 275 freshmen at Cornell University showed that parents are often unaware of their child's behavior and personality, especially in school. Most of those teenagers have double lives between the house and when they are outside of the house. In conclusion, this research paper shows that behavior and personality development have is greatly affected by a myriad of factors like peer, genetic, and environmental influences.