Research concerning the public health effects of pornography is inconclusive; whilst many studies indicate an inverse relationship between availability of pornographic material and the incidence of sex crime, some studies have shown no such correlation, or may indicate an increase in the rates of such crimes.
An epidemiological study describes the association between given behaviors or environmental conditions, and physical or psychological health by means of observation of real-world phenomena through statistical data. Epidemiological studies generally have high levels of external validity, insofar as they accurately describe events as they occur outside of a laboratory setting, but low levels of internal validity, since they do not strongly establish cause-and-effect relationships between the behaviors or conditions under study, and the health consequences observed. External validity is the validity of generalized (causal inferences in scientific studies usually based on experiments as experimental validity. Internal validity is the validity of (causal inferences in scientific studies usually based on experiments as experimental validity.
"The effects of Pornography: An International Perspective" was an epidemiological study which found that the massive growth of the pornography industry in the United States between 1975 and 1995 was accompanied by a substantial decrease in the number of sexual assaults per capita; and reported similar results for Japan. Findings of this nature have been critiqued by Robert Peters, president of Morality in Media, on the grounds that the results are better explained by factors other than the increased prevalence of pornography:
a more plausible explanation is that if there is a decline in “forcible rape,” it is the result of a tremendous effort to curb rape through community and school-based programs, media coverage, aggressive law enforcement, DNA evidence, longer prison sentences, and more. Morality in Media Inc ( MIM) is an American Non-profit, Interfaith organization that was established in New York in 1962 
Since Denmark was the first country to legalize pornography (in 1969), it was also the first place where researchers employed epidemiological methods in an attempt to assess any consequences of free access to pornography.
Danish criminologist Berl Kutchinsky's Studies on Pornography and sex crimes in Denmark (1970), a scientific report ordered by the Presidential Commisson on Obscenity and Pornography, found that the legalizing of pornography in Denmark had not (as expected) resulted in an increase of sex crimes. Berl Kutchinsky (1935 - 1995 was a Danish Professor of Criminology at the University of Copenhagen. The Kingdom of Denmark ( ˈd̥ænmɑɡ̊ (archaic ˈd̥anmɑːɡ̊ commonly known as Denmark, is a country in the Scandinavian region of northern Europe
Since then, many other experiments have been conducted, either supporting or opposing the findings of Berl Kutchinsky, who would continue his study into the social effects of pornography until his death in 1995. His life's work was summed up in the publication Law, Pornography, and Crime: The Danish Experience (1999).
In stark contrast to the previously described research, a review of epidemiological studies found that the quantity of pornographic material viewed by men was positively correlated with degree to which they endorsed sexual assault. 
In this context, a controlled study describes the correlation between given behaviors or environmental conditions and health effects in a laboratory setting in which conditions other than those under study are effectively held constant across groups of participants receiving various levels of the experimental condition(s). Since it is considered that the only functional difference between groups is the level of experimental condition(s) received, researchers can strongly infer cause-and-effect relationships from statistically significant associations between experimental condition(s) and health consequences. In Statistics, a result is called statistically significant if it is unlikely to have occurred by Chance. Thus, controlled studies have high levels of internal validity. Internal validity is the validity of (causal inferences in scientific studies usually based on experiments as experimental validity. However, such studies often suffer from questionable external validity due to the considerable differences between real-world environments and laboratory settings, and the consequent belief that results cannot be generalized beyond the experimental context. External validity is the validity of generalized (causal inferences in scientific studies usually based on experiments as experimental validity. A double-blind study involves participants who are unaware of the levels of experimental condition(s) that they are receiving, and recording of data by observers who are likewise not informed of the groups to which participants have been assigned. The blind method is a part of the Scientific method, used to prevent research outcomes from being influenced by either the Placebo effect or the Observer Double-blind studies avoid biased results due to participants' or observers' beliefs concerning the effects of the experimental condition(s). While often performed to assess the effects of pharmaceuticals, since placebo medication can be employed, double-blind studies concerning the health consequences of viewing pornographic materials are impossible, as participants would be aware of their exposure to the experimental condition. Placebo is a substance or procedure a patient accepts as medicine or therapy but which has no specific therapeutic activity
"Effects of Prolonged Consumption of Pornography" was review of controlled studies which found that extensive viewing of the type of pornographic material commonly sold at adult bookstores was positively correlated with leniency in the sentencing of a person convicted of rape in a mock trial setting (figure 5), decreased satisfaction of participants with their sex lives and partners (figure 10), and an increased self-reported willingness to commit rape or other forced sexual acts (figure 12). This line of research has been critiqued in "The effects of Pornography: An International Perspective" on external validity grounds:
Lab experiments typically do not take into account context and other crucial social and situational factors in considering the audience or the material. . . In real life, individuals are free to satisfy different sexual urges in ways unavailable to students in classroom or subjects in laboratory situations.