Recently, Michael Castleman made a general claim (seconded by Gad Saad) that there is little evidence of pornography-induced social harm. He backed up this assertion with some general statistics from the United States: "...as porn viewing has soared, rates of syphilis, gonorrhea, teen sex, teen births, divorce, and rape have all substantially declined. If Internet porn affects society, oddly enough, it looks beneficial."
This is, of course, a highly misleading argument. Why? Because other social, educational, and technological advances have accompanied the growth of the pornography industry. Contiguity of events is not an indicator of causality, only a precondition. (Crack cocaine use is down, too... Should we thank Larry Flynt?) It is possible that pornography consumption has null effects on these social problems; but it is also possible that pornography contributes to these problems in degrees that are compensated for by these other advances.
So if broad social trends won't cut it, what subtle effects might we look for? Here are a few off the top of my head, some of which have already been subjected to careful study...
Castleman made a brief reference to feminist sociologist Robert Jensen. I read Jensen's recent book Getting Off about two months ago. It's a worthwhile read, though written in an unflinchingly frustrated tone. (I'm paraphrasing here, but Jensen essentially says that it's time for men to walk the talk of treating women equally, and that walking the talk means curbing pornography consumption. I can imagine this is a bitter pill for some to swallow, whether because of personal consumption habits, values concerning free expression, or attitudes about gender equality. You can listen to an interview with Jensen here.)
Indeed, Jensen's claims that pornography may strengthen existing violent proclivities seem eminently reasonable. While it is hard to pin down broad social effects of pornography consumption in the way Castleman tried to do, a fair amount of recent research on pornography's effects examines how certain personality "vulnerabilities" may heighten pornography's influence.
A point of agreement: Castleman correctly noted that pornography makes a lousy instructional manual for pleasurable partnered sex. I would add that it is very hard to see how most pornography is instructive for contraception or disease control. Condom use is infrequent in these media -- supposedly because consumers prefer viewing "bareback" sex -- and demonstrations of other prophylaxes (e.g., dental dams for cunnilingus) are all but non-existent. Because the "money shot" (footage of ejaculation, usually on the woman's body or face) is expected in heterosexual male-oriented porn, coitus interruptus ("pulling out") is the name of the game. (Perhaps this is why teen birth rates are down, he said sarcastically!)
Furthermore, sex acts that carry higher risks of injury, humiliation, and/or disease contraction (e.g., multiple simultaneous penetrations; sadomasochist acts such as choking and slapping during intercourse; anal sex; penetrations followed immediately by oral sex) are depicted as routine. Indeed, the women often perform as if these behaviours are particularly pleasurable, selling the fantasy that every woman has a hidden "slut" switch, waiting to be flipped on by the right kink. Such normalization of unusual practices might heighten perceived conformity pressures -- from one's partner and from "society" -- to engage in sex acts that one finds uncomfortable.
(This is not an argument from prudishness, by the way. You could claim that pornography may encourage partners to experiment with novel and mutually enjoyable positions/practices. I'd counter that so would any decent book on sexual instruction, and without quite so much profitable degradation of women.)
The invisibility of safer sex practices, incidentally, is also a common concern among critics of "romance novels" aimed at women. In a neat pair of studies, Diekman et al. (2000) demonstrated that, first, regular consumption of romance novels was associated with reduced self-reported intent to use condoms, and second, the depiction of condom use within a romantic story context increased self-reported intent to use condoms.
In sum, pornography is not a fictional depiction of sex -- it is real sex embedded within a tissue of convenient fictions. If consumers with pre-existing psychological vulnerabilities model their own sexual acts upon what they see in typical heterosexual pornography, harmful expectations about sexual behavior may emerge.
Freedom of expression must be tempered with due concern about the meaning of those expressions. Given today's ubiquity of pornographic material, we should not be content with easy answers about its likely legacy.
Diekman, A. B., McDonald, M., & Gardner, W. L. (2000). Love means never having to be careful: The relationship between reading romance novels and safe sex behavior. Psychology of Women Quarterly, 24, 179-188.