Credit: Pixabay.

The motion picture content rating system is meant to classify films by how suitable they are for various audiences, particularly regarding things like sex, violence, substance abuse, or other types of mature content. Typically, most ratings around the world carry age recommendations, but these can be highly subjective in some cases — and this is where a new study comes in.

Researchers at the Max Planck Institute for Chemistry, Germany found a measurable criterion for determining the age ratings of movies. Their study suggests that concentrations of isoprene inside a movie theater — a gas released when people are nervous and tense — can be a good indicator for age ratings.

The smell of tension

In Germany, age ratings are classified by the Voluntary Self Regulation of the Movie Industry (FSK) committee, which examines the content of every motion picture available in the country. For instance, movies such as Harry Potter, Star Wars and Dracula are only suitable for viewers aged 6, 12, 16 or 18 respectively, according to the FSK rating.

Being scientists, the Max Planck researchers thought of a more objective method for rating a movie’s suitability for various audiences. During 135 screenings of 11 different movies, they installed highly sensitive mass spectrometers in the cinema’s ventilation system. The device is capable of tracking changes in the air composition at the parts per trillion (ppt) level. Every 30 seconds, the team analyzed the concentration of 60 different compounds.

The chemical that peaked their interest was isoprene, which is formed in the human body by metabolic processes and is stored in the muscle tissue. Whenever we move or tense, trace quantities are released through the expired air via the circulatory system.

“Evidently, we involuntarily squirm back and forth on our cinema seat or tense our muscles when we become nervous or excited,” Jonathan Williams, leader researcher at the Max Planck Institute for Chemistry, said in a statement.

The authors reported in the journal PLoS ONE that for a variety of movie genre and age groups, the isoprene levels reliably correlated with the age rating determined by the industry body.

“Isoprene appears to be a good indicator of emotional tension within a group,” Williams said. “Our approach could therefore provide an objective criterion for deciding how movies should be classified.”

The new method could prove useful in determining the age rating of a movie in disputed movies — it’s not always clear-cut which rating works best. It could also become a useful tool for tracking how age classification standards change over time.

Williams has another idea, too. He and colleagues plan to investigate correlations between other chemical compounds and other human emotional states, not just tension. Whether specific emotions leave traces in the air is the subject of another study in the future, which will require more controlled laboratory conditions.