The moratorium lasted for the 9 years between a Colorado execution in 1967 and a Utah execution in 1977. This 'natural phenomenon' was used to determine whether the absence of the application of the death penalty was associated with a significant rise in the rate of criminal homicides. The study used the average annual rates of criminal homicides in each of 40 States for two periods: 1958-1966 and 1968-1976. The homicide rate was significantly higher (p less than 0.001) in death penalty States after the moratorium. This seemed to indicate that capital punishment has at least some deterrent effect. However, an examination of the control group also revealed a significant difference (p=0.03) between premoratorium and postmoratorium homicide rates in the nonpenalty States. Thus, any rise in the homicide rates in penalty States was due mainly to an overall rise that was independent of the death penalty and its deterrence factor. The homicide rates in death penalty and nonpenalty States did not differ significantly. Table and 20 references. (Author abstract modified)