Stressing Demeanor Credibility: Continued Impacts of Melendez-Diaz for Forensic Scientists

Fri, 02/10/2012 - 2:38pm
Ronald K. Bullis, Ph.D., J.D.

It is only shallow people who do not judge by appearances. –Oscar Wilde, Picture of Dorian Gray

Forensic scientists begin their testimony even before they speak their first words. Even before they take the oath or recite their qualifications, the jury is assessing their credibility by their demeanor. Simply put, demeanor evidence is the body’s compass, pointing in the direction of credibility. It includes gestures, intonations, posture, mannerisms, eye movements, inflections, and expressions. Judges and juries listen and look very closely to demeanor evidence to assess the credibility of forensic testimony. Research has repeatedly shown that demeanor evidence significantly determines court decisions.1 Recently the U.S. Supreme Court has reinforced the importance of demeanor evidence in the 2009 case of Melendez-Diaz v. Massachusetts.2

That decision required forensic scientists to personally testify to their research and conclusions. The Melendez-Diaz Court based its decision on the 6th Amendment’s Confrontation Clause and stressed the essential practice of jurors to see and to hear people testify, not just examine affidavits. Forensic Magazine has tracked the progress and discussed the significance of the Melendez-Diaz case for forensic scientists.3 Before Melendez-Diaz, it was common practice for forensic scientists to submit their results by affidavits. This will no longer be the standard practice. Forensic science testimony now requires as much attention to the manner in which they testify as to their scientific research. It is no longer enough to be good scientists, forensic experts need to be effective communicators.

Because it is unlikely they will have had testimony training or education in their formal schooling, forensic scientists will need to get additional training, particularly in communication and persuasion skills, human relations, and relaxation techniques.

Demeanor Evidence and Forensic Scientists
While research shows that demeanor evidence immediately and significantly impacts the jury’s evaluations of credibility, it is the “Cinderella” of forensic science. Its significance is often overlooked and ignored. Demeanor is left holding the mop and broom when her sisters go off to the ball to thrill in the dance of DNA and digital evidence. Melendez-Diaz has helped reinvigorate the significance of demeanor evidence. That case’s majority opinion made clear that the Confrontation Clause intended to help judges and juries to distinguish between accurate, clear, and forthright testimony and testimony that misses that standard.4 At the same time, neuroscience and behavior scientists continue to show the importance of witness demeanor for legal decisions. Demeanor is the Prince Charming that can sweep both Cinderella and the jury off their feet.

This analogy might be hard to believe. After all, isn’t law about cold, hard facts? Don’t juries want to hear the cool, linear logic of hard scientific research from forensic scientists when considering the evidence? The answer is that judges and jurors rely on demeanor evidence every bit as much as in hearing facts and in logically considering them.5 Legal researchers, social scientists, and neuroscientists are studying how courts and juries use demeanor evidence to assess credibility. Because jurors have used demeanor evidence from the beginning of our judicial system and Melendez-Diaz now often requires personal testimony, forensic scientists will need training to comport their personal demeanor to their scientific deliberations.

The first lesson in demeanor evidence is its long-standing legal tradition. The Melendez-Diaz case did not arise from nowhere. Western justice has long used demeanor evidence to prove guilt, albeit not always in ways we would now approve. In 1050 c.e. Lady Emma, twice crowned Queen of England, was accused of adultery with the Archbishop of Canterbury. To test her deception or innocence, she had to walk barefoot over nine red-hot plowshares. If her skin was undamaged, she was innocent. But if her feet were scalded, she would be hung. Her son, the King, was already busy claiming her properties and rescinding her titles. To everyone’s amazement, however, she walked across the plowshares unharmed and her repentant son begged for forgiveness.6 At least that’s what legend says.

Thankfully, today’s courts value demeanor even more, but apply it differently. The U.S. Supreme Court in U.S v. Scheffer7 (1997) said that juries are “lie detectors” and another Supreme Court ruled that credibility in witness assessments includes tone of voice and variations in behavior.8 Jurors use witness demeanor as human technology, not to detect lies in forensic experts, but to determine both scientific truth and persuasive truth.

New Insights from Legal Research and Neuroscience
Both neuroscience and juror research reveal helpful insight for expert witnesses and this article will apply them to forensic scientists. First, telling scientific truth and testifying to persuasive truth is not the same thing. Being technically correct is not the same as being persuasively competent. Forensic scientists must distinguish the credibility of their own testimony from the conclusion of their opposite number. After all, qualified forensic scientists on both sides will testify to opposing scientific truth. Juror research teaches that demeanor credibility consistently ranks as a significant factor in juror persuasion.

Second, demeanor evidence is important because it is how humans communicate and make decisions. One of the lessons of neuroscience is that our demeanor says much more than our words. Scholars estimate, for example, that at least 80% of our communication is nonverbal.9 Everyone notices the processes of communication more quickly and more efficiently than the content of communication. Jurors generally find how a forensic expert conveys their research equally compelling as the results themselves. Words conveying information came later in our evolutionary stage than physical expression. Our prehistoric ancestors had to quickly scan the environment for predators. In fact, a large part of our demeanor assessments are conducted in the brain’s most ancient regions.10 So, demeanor evidence is an instinctual part of our behavior.

Yet, our demeanor assessments are not always correct. Evidence shows that judges and juries not only rely upon demeanor evidence in assessing witness credibility, they report that they are pretty good at it.11 Jurors will depend more upon non-verbal cues to assess credibility as expert credentials and forensic technology become more complex and standardized. Forensic scientists will likely face increasingly sophisticated demeanor testimony as more and more scientists are called to testify. They serve justice by presenting competent demeanor evidence.

Third, not only do judges and jurors scrutinize witness demeanor, but they do so almost instantaneously. Research has shown that our brains process demeanor as quickly as 100 milliseconds. 12 This means that jurors size up experts before they utter their first word or even take the stand. Jurors will begin assessing the forensic scientist even as he or she walks into the courtroom or takes the oath. Those immediate impressions tend to last throughout deliberations.

Fourth, research also shows that juries find the eyes of a witness to be the single most important visual cue to assess credibility.13 In one case, a defendant insisted upon wearing sunglasses at the trial and unsuccessfully appealed the conviction based upon how the sunglasses may have undermined the testimony.14

Fifth, a confident witness is more persuasive than a hesitant one.15 Acting confidently, even passionately helps establish credibility. One study found that jurors are twice as likely to trust a confident expert over an uncertain or unsure expert16 or even three times as much.17

Certainty has its persuasive advantages— and its pitfalls. Arrogant experts rarely gain credibility. Arrogance strikes jurors as annoying and possibly rigid and single-minded in their scientific work. In fact, studies show that confident and arrogant demeanors can be culturally-sensitive behaviors. Women who act like the legal system thinks they should act when they file sexual harassment claims are more likely to be believed.18 One’s assessment of the “right” demeanor of “virtuous” women is culturally dependent. Right or wrong, we often assess others’ credibility by our personal standards.

In fact, culturally-sensitive testimony in general is most likely to be interpreted as more persuasive. 19 A classic example is looking a person in the eye. Many Westerners expect persuasive speakers to look them in the eye. However, lowering of the eyes in some cultures, including Native American and many Eastern cultures, is a sign of respect and civility. Similarly, the feet in Middle Eastern cultures are a sensitive part of the body. Showing the soles of one’s feet to another is a grave insult. Offending jurors is unlikely to help them be good listeners.

Demeanor Evidence and Persuasive Forensic Testimony
The following are suggestions for forensic scientist to consider when testifying. This brief inventory flows from legal and scientific research on the brain and in juror research. These can be used alongside with the suggestions by Ashlock noted in Forensic Magazine20 and are not intended to be exhaustive.

1. The expert is his or her testimony. Jurors do not discriminate between the demeanor of witnesses and their testimony.21 The content of testimony and the process of that testimony comes whole and intact to the juror’s attention. We instinctively and pervasively look for clues of credibility in another’s demeanor. Forensic scientists who take seriously how they testify just as much as what they testify about are the most effective witnesses. The Melendez-Diaz case underscores how seriously the law takes demeanor evidence. Valuing the role of demeanor evidence is the first lesson in effectively cultivating it.

2. Forensic scientists need to act like their jury testimony begins the second they enter the courtroom. Because it does. The jury assesses their testimony as soon as they see them enter the courtroom. Experts should be ready to testify in court the instant they are called by the bailiff or enter the deposition room. They say that General George S. Patton put on his “war face” when meeting with his soldiers and other officers during battle. Forensic scientists should likewise put on their “forensic faces” as soon as they enter the courtroom or are deposed. It should be a courteous face, professional, but expressive, even assertive. Their “forensic face” expresses eagerness, even excitement over explaining and sharing their research and conclusions.

3. Passion is good. Propaganda is not. Jurors want to see that the expert cares about his or her testimony, but don’t like robotic, bored, or pro forma answers.22 Firm and assertive testimony will inject persuasive inflection and emphasis into the tone, vocabulary, inflection, and cadence of testimony. Yet, rigid, arrogant, and rote testimony, accompanied by a raised voice, arm-waving, or pointing is both distracting and appears pre-packaged or pre-arranged.23

4. Forensic experts should tailor their testimony for a demographically diverse jury. The scientists’ legal team and consultants can help describe the jury so that the testimony can speak most persuasively to a culturally-diverse jury. Using this information, the scientists can shape their testimony to testify clearly and completely.

5. For many, if not most jurors, the eyes of witnesses are, indeed, “the mirrors of the soul.” Jurors pay close attention to how jurors use their eyes. Even when some cultures find direct eye contact challenging, even defiant, they would find a witness who never looks anyone in the eye, whose eyes wander distractedly or aimlessly around the room, or whose eyes are covered by hair, clothing, or accessories to be disconcerting. Avoid wearing clothing that distracts the jury from viewing your face, especially your eyes.

6. Cultivate testimonial humility. This means a style that is authoritative, but not authoritarian, neither deferential nor defiant. Avoid disparaging remarks or a patronizing tone when disagreeing with your opposite number’s conclusions. At the same time, scientists can explain and defend their own conclusions firmly and courteously.

7. While it is desirable for expert witnesses to let their individual personality show, that rule is not dispositive. Experts should always tailor their personal traits to the service of credibility. Mannerisms, expressions, vocabulary, and all that makes up demeanor evidence should be tailored to the service of communicating good science and sound, effective testimony. Personal traits are no longer personal when the expert takes the witness stand.

The best message comes from the best messenger. The scientific evidence put forth at trial is only as effective as the scientist who presents it. Jurors consistently and significantly use demeanor evidence to determine credibility. Because jurors face scientific testimony from opposing experts, they use demeanor evidence to determine which opposing testimony seems more authentic and plausible. Jurors use the body’s compass to determine which witness to find most credible. Melendez-Diaz underscores that legal mandate and intensifies the need for demeanor training for forensic scientists so they can testify just as effectively.

Forensic scientists don’t just offer science to a jury. They offer a human being. All-in-all, the single, best testimony an expert can offer the jury is the expert themselves.


  1. Timony, J. (2000). Demeanor credibility. Catholic University Law Review, 49, 903.
  2. Melendez-Diaz v. Massachusetts, 129 S. Ct. 2527; 174 L. Ed. 2d 314; U.S. LEXIS 4734 (2009).
  3. Bullis, R. (2011). Applying Melendez: Briscoe and beyond. Forensic Magazine, 8(3), 15-19; Craig, C. Confronting science: Melendez-Diaz and the Confrontation Clause,, posted September 17, 2010.
  4. Melendez-Diaz at 2009 U.S. LEXIS 4734 at 10-14.
  5. Simon, C. (2011). The limited diagnosticity of criminal trials. Vanderbilt Law Review, 64, 143-223; Minzner, M. (2008). Detecting lies using demeanor, bias and context. Cardozo Law Review, 29, 2557 and Timony, J. supra at 1.
  6. Shea, A. & Van Aperen, S. (2006). The truth about lies. ABC Books.
  7. Simon, supra note 5 cites U.S v. Scheffer, 523 U.S. 303, 313 (1997).
  8. Simon, supra note 5 cites Anderson v. Bessemer City, 470 U.S. 564, 575 (1985).
  9. Moss, M. (n.d.). Memory. (audio lecture). Institute for Brain Potential. Haddonfield, NJ. and Shea and Van Aperen, S. supra at note 6.
  10. Adolphus, R. (2002). Trust in the brain. Nature Neuroscience, 5, 8-9 in Porter, S. & ten Brinke, L. (2009). Reading between the lies: identifying concealed and falsified emotions in universal facial expressions. Psychological Science, 19, 508-514.
  12. DePaulo, B. et. al.(1997). The accuracy-confidence correlation in the detection of deception. Personal and Social Psychological Review, 1, 346.
  13. Willis, J. & Todorov, A. (2006). Making up your mind after a 100-ms exposure to a face. Psychological Science, 17, 592-598.
  14. Vrij, A. (2008). Detecting lies and deceit: pitfalls and opportunities. Chichester, UK: Wiley.
  15. Morales v. Artuz, 281 F.3d 55; U.S. App. LEXIS 2179 (2002).
  16. Lindsay, R. (1994). Can people detect eyewitness-identification accuracy within and across situations? Journal of Applied Psychology, 79, 83.
  17. Wells, G. (1984). How adequate is human intuition for judging eyewitness testimony? In Eyewitness Testimony: Psychological Perspectives 256, 254 (Gary Wells & Elizabeth Loftus, eds.)
  18. Easteal, P. & Judd, K. (2008). ‘She said, he said’: credibility and sexual harassment cases in Australia. Women’s Studies International Forum, 31, 336-344.
  19. Porter & Brinke, supra, note 10 at 24; Vrij, A. (2000). Detecting lies and deceit: the psychology of lying and the implications for professional practice. Chichester, UK: Wiley.
  20. Ashlock, S. Expert witness: effective courtroom testimony. posted February 1, 2010.
  21. Whitman, M. (1999). Communicating to capital juries: how life versus death decisions are made, what persuades and how to most effectively communicate the need for a verdict of life. Capital Defense Journal, 11, 263.
  22. Gabriel, R. (2009). Redefining credibility: turning expert witnesses into teachers. The Jury Expert, 21(3), 85-92.
  23. Cannon, D. What preparation does your expert need? The Jury Expert, 21(3), 93-95.

Dr. Ronald K. Bullis’ next book will be The Narrative Edge: A guide for social work and mental health expert witnesses. Contact him at and see more about his work on


Share this Story

You may login with either your assigned username or your e-mail address.
The password field is case sensitive.