Physiognomy (from the Greek physis meaning "nature" and gnomon meaning "judge" or "interpreter") is the practice of assessing a person's personality (and other mental characteristics more generally) from their outer physical appearance, especially the face. The term tend to refer to outer appearance characteristics that can be quickly identified from visual inspection, and not to characteristics that may require careful measurements, such as in craniometry.
While some early claims regarding physiognomy were problematic, more recent research have found various associations between outer appearance and mental characteristics.
One aspect is that the sex of person can be identified from outer appearance and various associated measured differences between the sexes.
Gaydar (from gay + radar) refers to the intuitive ability of a person to assess others' sexual orientations as gay, bisexual or heterosexual. Gaydar relies on verbal and non-verbal clues and LGBT stereotypes. See also Homosexuality: Similarities between homosexual men and heterosexual women.
In 2017, a study stated that an AI image recognition algorithm could detect sexual orientation more accurately than humans (in 91% of the tested cases for men and 83% for women, if using several photographs of the same individual). The study stated that "Consistent with the prenatal hormone theory of sexual orientation, gay men and women tended to have gender-atypical facial morphology, expression, and grooming styles."
See Political spectrum: Masculinity and femininity on this topic.
Several facial features have been argued to be related to testosterone levels during development and aggressive behaviors.
Another aspect is race, which can be identified from outer appearance and is associated with various characteristics, as discussed in the various race articles.
Jewdar is the argued ability of a person to be able to detect or intuitively sense whether another person is a Jew or Jewishness more generally.
A 2014 study stated that "Minor physical anomalies (MPA) are neurodevelopmental markers, which manifest as unusual morphological features on the face and body. MPAs during childhood can be regarded as a predictor or risk of neurodevelopmental disorders because both the skin and brain are derived from the neuro-ectoderm during the first three months of fetal life. These minor anatomical anomalies may parallel early developmental defects of the central nervous system and may possibly stem from prenatal insults or congenital abnormality. The discrete signs of MPA, e.g., small head circumference, hypertelorism, malformed ears, high palate, epicanthus was found on children with Down's Syndrome, or congenital neurodevelopment disorder. According to literature review, children with autism, autism spectrum disorder, delayed speech, learning disabilities, mental retardation, hyperactivity, rebellious nature, destructive behaviour, criminality, living in an adverse family environment or idiopathic retardation, school failure, schizophrenia and autism, or some temperament problem had reported to have more signs of MPA than those children without such abnormalities. MPAs are prevalent with an average of 15% of normal population experiencing some MPA and are easily measured by examination."
- Deep neural networks are more accurate than humans at detecting sexual orientation from facial images. https://osf.io/zn79k/
- Facial Profiling http://www.slate.com/articles/health_and_science/science/2009/10/facial_profiling.single.html
- A Pilot Study: Association between Minor Physical Anomalies in Childhood and Future Mental Problems https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4124179/