The unexplored confound

First page of an underground porn comic, c. 1930s (Wikicommons). Pornography is now much more available and better in quality.

My last post presented a German study whose findings suggest that prolonged exposure to porn atrophies those portions of the male brain that process erotic stimuli (Kühn and Gallinat 2014). On the other hand, the arrow of causality might point the other way. Perhaps a man will seek out and view more porn if he already has less of the gray matter for sexual arousal.

The jury is still out. In this debate, we should keep in mind that people have argued against porn for different reasons. Refuting one argument doesn't necessarily refute the others.

Historically, porn has been condemned for three reasons:

It incites men to commit rape and other forms of sexual abuse. This is the oldest argument and is still used.

It objectifies women and causes men to treat women with less respect. This is the feminist argument of the 1960s and 1970s.

It desensitizes men to erotic images and causes them to seek more porn and harder porn to achieve the same effect. This argument is recent.  

The first argument has always been problematic. A recent review article concludes that "[i]t has been found everywhere it was scientifically investigated that as pornography has increased in availability, sex crimes have either decreased or not increased" (Diamond 2009).

The second argument likewise seems weak. A Danish survey found that porn actually seems to improve male attitudes toward the opposite sex:

The self-perceived effects of "hardcore" pornography consumption were studied in a large representative sample of young adult Danish men and women aged 18-30. Using a survey that included the newly developed Pornography Consumption Effect Scale, we assessed participants' reports of how pornography has affected them personally in various areas, including their sexual knowledge, attitudes toward sex, attitudes toward and perception of the opposite sex, sex life, and general quality of life. Across all areas investigated, participants reported only small, if any, negative effects with men reporting slightly more negative effects than women. In contrast, moderate positive effects were generally reported by both men and women, with men reporting significantly more positive effects than women. (Hald and Malamuth 2008)

Of course, this is self-report, which often reveals not what people think but rather what people think they're supposed to think. Self-report showed that Trump was going to lose the 2016 election. In a liberal society, male respondents might think twice before saying their attitudes toward women have worsened. For this reason, it's interesting that similar results have been obtained from a sample of university students in Indonesia—a conservative, Muslim majority country with strict anti-pornography laws (Mulya and Hald 2015).

With regard to the third argument, two American studies have presented findings that seem to contradict those of the German study. In the first one, male and female participants were exposed to erotic and neutral images, and their neurological response was measured during the first second of exposure. Hypersexuals (heavy consumers of porn) showed no signs of habituation or desensitization to erotic images in relation to neutral images (Prause et al. 2015).

The other American study likewise found no signs of desensitization:

Data from a large sample of men (N = 280) across similar studies were aggregated to test the hypothesis that consuming more VSS [visual sexual stimuli] was related to erectile problems. These men answered questions about their sexual behaviors and feelings, including their consumption of VSS, and viewed sexual films in the laboratory. Those who reported viewing more VSS in their own life reported higher sexual arousal to films in the laboratory. Self-reported erectile functioning with a partner was not related to the hours of VSS viewed weekly. Finally, those who viewed VSS more also reported higher desire for both partnered sexual behaviors and solo sexual behaviors. This pattern suggests that those who view more VSS likely have a higher sexual drive and experience a stronger sexual response to standardized VSS than those who view less VSS. Sexual arousal responsivity may not be impaired by viewing more VSS at home, as it actually was related to stronger desire and sexual arousal in two of the three relationships tested. (Prause and Pfaus 2015)

Some of these findings are compatible with those of the German study. The latter study looked for long-term effects of porn consumption and found evidence of atrophy in some areas of the brain. Viewing porn in a laboratory won't produce any measurable atrophy because the time spent viewing isn't long enough (the films varied in length from 20 seconds to three minutes). The same kind of objection holds for the other American study discussed above, which found that porn consumption didn't reduce sexual arousal during the first second of exposure to an erotic image. Perhaps porn consumption makes sexual arousal less sustainable even though the initial response remains as strong as ever.

On the other hand, it's harder to dismiss the self-report data from the second American study: men who viewed more porn had a higher level of sexual desire than those who viewed less. So what gives?

Perhaps the two groups of men were different to begin with. This is plausible because the pool of participants was much more diverse in this American study than in the German study. The latter study excluded participants with psychiatric, medical, and neurological disorders or with substance abuse problems, whereas the American study had much weaker exclusion criteria. Furthermore, the German study had German participants, whereas the American study had participants who were 53.3% white, 23.1% Hispanic, 16.0% black, and 7.6% other or unknown (Kühn and Gallinat 2014; Prause and Pfaus 2015).

No one seems to have considered that ethnic/racial background might be confounded with sexual arousal or hours spent viewing porn. It's not as if this kind of confound is unlikely. Brown and L'Engle (2009) found higher porn consumption by young African Americans than by young European Americans. Ybarra and Mitchell (2005) reported that "Hispanic youth were almost three times as likely to report online seeking versus offline seeking behavior versus otherwise similar youth of non-Hispanic ethnicity (p = .02)." Price and Miller (1984) found that African Americans were also more likely than European Americans to use sexual fantasy to achieve arousal.

Just as importantly, according to a recent review article (Collins et al. 2011), erotic content in music, movies, and magazines seems to have different impacts on the development of sexual behavior in young white and black Americans:

Brown and colleagues subsequently expanded on this work by linking exposure to sexual content in a broader variety of media to intercourse initiation and advances in noncoital behavior. They surveyed 1,017 North Carolina youth when they were 12-14 years old and again two years later. Sexual content exposure in television, music, movies, and magazines predicted advancing sexual behavior, even after other variables were controlled for statistically, but only among white youth, who comprised about half of the sample. No relationship was observed among African-American teens, who made up the other half of the study sample. 

Most recently, Hennessy and colleagues analyzed web surveys of 506 Pennsylvania teens aged 14-16 years at baseline and followed them annually for a total of three surveys. They examined television, music, movies, magazines, and video games with a sexual content exposure measure. Data were analyzed using growth curves, testing whether changes in exposure to sexual media over time are correlated with changes in sexual behavior during the same period. They found that changes in exposure to sexual content were associated with changes in behavior among white teens (the r = 0.46 correlation just missed statistical significance, perhaps due to the small sample), but there was no association among African-American youth.

Whether these differences between white and black Americans are innate or learned is beside the point. These differences exist, and they should be controlled in any study with a mixed pool of participants. Otherwise, it’s no longer clear which way the arrow of causality points. In this case, the American research team found that porn consumers have a higher level of sexual desire. They concluded that porn consumption increases a man’s desire for sex. Well, perhaps. Or perhaps there is a subset of men who consume lots of porn because they have a stronger desire for sex.

In any study of this sort, the pool of participants should be as homogeneous as possible, i.e., it should be limited to people who probably shared the same potential for sexual arousal when some of them began viewing porn.

Conclusion

Research on the harmful effects of porn is shifting to the hypothesis that excessive consumption desensitizes the male brain. It seems, however, that this effect varies from one man to another, both within racial/ethnic groups and between them. It may be that porn desensitization is a greater problem for men whose ancestors were overwhelmingly monogamous, i.e., from Eurasia. For them, porn has created something entirely novel that their ancestors never experienced and never had to adapt to: a virtual environment where a man can have sex with as many women as he wishes. In contrast, porn may be less problematic for men with ancestors from sub-Saharan Africa, where polygyny has long been the norm (Goody 1973; Pebley and Mbugua 1989).

References

Brown, J.D., and K.L. L'Engle. (2009). X-rated: Sexual attitudes and behaviors associated with U.S. early adolescents' exposure to sexually explicit media, Communication Research, 36, 129-151.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.664.4770&rep=rep1&type=pdf

Diamond, M. (2009). Pornography, public acceptance and sex related crime: a review, International Journal of Law and Psychiatry, 32(5), 304-414.

https://www.geekparadize.fr/wp-content/uploads/2016/08/pdf-86.pdf

Collins, R.L., S.C. Martino, and R. Shaw. (2011). Influence of New Media on adolescent sexual health: evidence and opportunities, Working Paper, Rand Health, April 2011

https://www.researchgate.net/profile/Rebecca_Collins8/publication/265231336_Influence_of_New_Media_on_Adolescent_Sexual_Health_Evidence_and_Opportunities/links/54f4cd9b0cf2eed5d7359159.pdf

Dupanloup, I., L. Pereira, G. Bertorelle, F. Calafell, M.J. Prata, A. Amorim, and G. Barbujani. (2003). A recent shift from polygyny to monogamy in humans is suggested by the analysis of worldwide Y-chromosome diversity, Journal of Molecular Evolution, 57, 85-97.

http://www2.webmatic.it/workO/s/113/pr-457-file_it-Journal%20of%20Molecular%20Evolution%2057.pdf

Goody, J. (1973). The Character of Kinship, Cambridge: Cambridge University Press.

Hald, G.M. and N.M. Malamuth. (2008). Self-perceived effects of pornography consumption, Archives of Sexual Behavior, 37(4), 614-625.

https://www.researchgate.net/profile/Gert_Hald/publication/5988339_Self-Perceived_Effects_of_Pornography_Consumption/links/0912f507bc978a257e000000/Self-Perceived-Effects-of-Pornography-Consumption.pdf

Kühn, S. and J. Gallinat. (2014). Brain structure and functional connectivity associated with pornography consumption. The brain on porn, JAMA Psychiatry, 71(7), 827-834. http://jamanetwork.com/journals/jamapsychiatry/fullarticle/1874574?=  

Mulya, T.W. and G.M. Hald. (2014). Self-perceived effects of pornography consumption in a sample of Indonesian students, Media Psychology, 17(1), 78-101.

http://www.tandfonline.com/doi/abs/10.1080/15213269.2013.850038

Pebley, A. R., & W. Mbugua. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (ed.) Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.

Prause, N. and J. Pfaus. (2015). Viewing sexual stimuli associated with greater sexual responsiveness, not erectile dysfunction, Sexual Medicine, 3(2), 90-98.

http://www.medsci.cn/webeditor/uploadfile/201503/20150321110618701.pdf

Prause, N., V.R. Steele, C. Staley, D. Sabatinelli, and G. Hajcak. (2015). Modulation of late positive potentials by sexual images in problem users and controls inconsistent with "porn addiction," Biological Psychology, 109, 192-199.

https://s3.amazonaws.com/academia.edu.documents/44169223/Modulation_of_late_positive_potentials_b20160328-20967-7vvhnt.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1507835633&Signature=buIAnKDHGZ66%2B7JKhw8K3ldMnIQ%3D&response-content-disposition=inline%3B%20filename%3DModulation_of_late_positive_potentials_b.pdf 

Price, J.H. and P.A. Miller. (1984). Sexual fantasies of Black and White college students, Psychological Reports, 54(3), 1007-1014.
http://journals.sagepub.com/doi/abs/10.2466/pr0.1984.54.3.1007

Ybarra M. and K.J. Mitchell. (2005). Exposure to Internet pornography among children and adolescents: a national survey, CyberPsychology & Behavior, 8(5), 473-486.

https://innovativepublichealth.org/wp-content/uploads/2009/04/cpb.2005.8.pdf