It seems logical that biological and psychological factors might explain why some people commit crime. but if crime is based on individual traits, how would we explain the fact that crime rates are higher in the west and south than in the midwest and east?

Psychological and biological factors are the main driving factors for the crime. Geographical factors have to be considered, as well. People living in the same area have similar morals and ethics. Indeed, psychology doesn’t have a significant effect in that sense. When taking the geographical factors into account, we can observe that crime rate is higher in the south and west. 

In 1958, the South had a homicide rate of nine per 100,000 compared to three per 100,000 for the U.S. One approach suggests that people in the South try to settle the disputes with the help of violence. Southerners pass this "culture of honor" behavior from generation to generation which it explains the biological aspect of violence. They honor to take the justice with their own hands.  

Poverty and hot weather are two other additional factors of violence in the South. South states, including Mississippi, West Virginia, Arkansas, Alabama, and Kentucky have got the lowest per capita income. How weather also makes people more aggressive, forces them to hit the streets and commit a crime.