“… everyday, spontaneous sexual behavior is a quintessentially private activity not open to public scrutiny.“

But not always.

As evidenced by the work of an investigatory team from the Facultad de Psicología, Universidad Autónoma de Madrid, Spain, where Jose-Miguel Fernández-Dols, Pilar Carrera and Carlos Crivelli “… analyzed the facial behavior of 100 volunteers who video-recorded their own expressions while experiencing an episode of sexual excitement that concluded in an orgasm, and then posted their video clip on an Internet site.”

Using the standardised Facial Action Coding System (FACS) the team identified ten ‘Action Units,’ (or ‘AUs’ in the FACS terminology) which were consistently prevalent in the 100 videos which were analysed.

• AU42 (slit eyes)
• AU43 (closed eyes)
• AU4 (frown/brow lower)
• AU6 (cheek raise)
• AU10 (upper lip raise)
• AU12 (lip corner pull)
• AU25 (lips part)
• AU26 (jaw drop)
• AU27 (mouth stretch)
• AU 29 or 30 (jaw thrust, or jaw sideways)

Nevertheless, despite these clarifications, the team's paper ends with a question : “Are frowns, scowls, and grimaces – either in pleasure or displeasure – mere expressions of muscular tension, blank displays that take psychological meaning from the perceiver’s rather than the sender’s mind?”

The study is published in the Journal of Nonverbal Behavior, Volume 35, Number 1, 63-71

Notes and caveats from the paper

“It could also be argued that expressions could have been faked or influenced by extrinsic factors …”

“… the individuals in our sample were self-selected, and we cannot exclude a potential bias due, for example, to the prevalence of some personality traits in our senders (extroversion?).”