One aspect of the new wave of expertise is credibility expertise, such as lie detector tests, psychiatric opinions on credibility, and truth serum tests. Another area of expertise focuses of comparison tests of items such as bite marks, tool impressions, and blood. Forensic dynamics, reconstructing movements, mechanics, and pathology from the resultant evidence have also been subjects of expert testimony in criminal trials. Other areas of expertise manifested in expert testimony involve mental processes, including hypnosis and psychiatric opinion on a person's mental state, and analysis of circumstances, such as expert testimony on the battered child syndrome and gang culture. Some of these new forms of expert testimony are admitted routinely in criminal cases; others are usually rejected; and others meet an uncertain fate. Courts generally refuse to allow experts to testify about their opinions of another witness's credibility. Most courts allow expert testimony concerning a medical condition, emotional disturbance, or psychological abnormality that impaired a witness's ability to perceive, recollect, or relate facts.