There has been significant investment in shares market in Nigeria in recent years. This paper investigates the factors that have influenced the share investment decisions of a sample of 2000 Nigerian investors. Anchored on the behavioral finance theory, it explored and analysed the influence of various social, economic, psychological, and cultural factors and sub-groups as indicated by available literature. Our findings showed the degree of influence of each factor as well as the sub-groups. However, there were no significant differences among the subgroups. The study therefore concluded that investment decisions in shares by Nigerians investors are influenced jointly by several groups of factors.