Genetics probably contribute a fair amount to our general level of happiness, up to 50 percent according to a 2005 review by Sonja Lyubormirsky, Kennon Sheldon, and David Schkade. However, this figure should be considered with some caution. Most genetic studies use samples that do not vary dramatically in their environment or life circumstances. When people’s environments are fairly similar, the influence of genetics is heightened. When there is a broad range of environmental conditions, however, the role of genetics is less important. Therefore, because most genetic studies do not include samples from a wide range of environments, the importance of genetics may be exaggerated.