I usually trust Paul W. Bosland (http://aces.nmsu.edu/academics/pes/paul-w-bosland.html) to give accurate info on pepper growing, http://aces.nmsu.edu/pubs/_h/H237/
The heat level in chile peppers is the result of two factors: the plant's genetics and the interaction of the plant with the environment. The genetic control of heat allows plant breeders to produce a chile pepper plant with a certain relative heat level. For example, the cultivar 'NuMex Joe E. Parker' was genetically selected to produce fruit of "medium" heat. However, environmental factors such as temperature and water influence the heat level. A mild chile pepper cultivar bred for low levels of heat will become hotter when exposed to any type of stress in the field. Conversely, a relatively hot cultivar given optimal environmental conditions will become only moderately hot. A chile pepper plant that genetically produces low-heat fruit will not produce hot chile peppers even when grown in a stressed environment. To produce chile peppers of a predictable heat, both cultivar selection and optimum stress-free growing conditions are important.