Because facial expressions trigger emotional responses so easily, scientists have used photos of emotional faces for a wide variety of brain imaging studies and other research.
Here is a sampling:
One part of the brain, an almond-shaped node known as the amygdala, is exquisitely sensitive to emotions, particularly negative ones. Several studies have shown it is more active when people view angry or fearful faces.
At the University of California at Los Angeles, developmental psychologist Nim Tottenham has taken a special interest in children adopted by American families from overseas orphanages.
Dr. Tottenham, whose mother is Korean, had been hearing reports that many of these children were having trouble controlling their emotions in stressful situations, despite being raised by highly involved, educated and often affluent parents.
When she tested those children around age 10 in a brain scanner, she found that their amygdalas were more active when they saw pictures of threatening faces than those of children who had not spent their first year or two in an orphanage.
The adopted children also had trouble making eye contact with those faces, she said, which may have been a tactic they employed to try to calm themselves down, because the eyes are so expressive.
At the City University of New York's John Jay College, neurophysiologist Jill Grose-Fifer has found a similar tendency in teenagers.
When she tested teens with trios of photographs of happy and scared faces and asked them to identify the expression in the middle, they took significantly longer than adults to identify a happy face when it was displayed between two scared ones because the scared faces were more distracting to them.
Studies also have shown that when teens are shown pictures of neutral faces, they often tend to see anger or other negative expressions in them.
As the mother of two teenage sons, Dr. Grose-Fifer has experienced this herself.
"I know that I can be concentrating and my sons will say, 'What's wrong?' and I'll say 'Nothing's wrong,' but because you've got a neutral face, they're perceiving that something's not right.
"That might explain some teenage angst -- you know, 'My mom was really mad at me,' but she wasn't mad at you, she just wasn't smiling."
Angry faces even can have an impact on us at the subconscious level, Harvard University researcher Jorge Almeida has found.
Using red-green "3-D" eyeglasses, Mr. Almeida first showed college students subliminal pictures of different faces in one eye and random patterns in the other.
He then showed pictures of Chinese word characters and asked the students how much they liked each one. They tended to say they disliked the ones that had been preceded by an angry face, even though they had never consciously seen the expression.
The phenomenon seems to involve an older part of the brain that routes negative emotional information to the amygdala on a separate pathway from the one normally used to process visual information, he said.
The negative-face technique also can be used to test whether people have blunted emotional reactions.
In a recent study, University of Pittsburgh psychiatrist Eydie Moses-Kolko found that mothers suffering from postpartum depression showed less reaction to fearful faces in a brain scanner than women who weren't suffering from the disorder.
Not reacting to pictures of fearful faces, Dr. Moses-Kolko said, is particularly telling in postpartum depression because "infants have a limited repertoire of how to communicate what they want, and facial expressions are one of their ways of doing that."
In 2001, Dacher Keltner and LeeAnne Harker at the University of California at Berkeley found that the types of smiles women college graduates showed in their yearbook photographs paralleled how well their lives had turned out 30 years later.
Using a longitudinal study of graduates from Mills College in California, the researchers rated how genuine and intense the smiles were in the yearbook photos, using a measurement system developed by former Berkeley professor Paul Ekman.
They also had access to personality profiles of the all-female graduates over decades, along with information about their marital histories.
They found that the more genuine a yearbook smile was, the more positive the woman's personality was even years later, and the more likely she was to be married and never divorced.
"It may be," they speculated, "that the women who smiled in intense fashion may consistently experience more positive emotion, and it is this ... that accounted for their personality ratings and life outcomes."
Some research suggests that our facial expressions aren't just the result of our feelings, but feed back information to the brain that intensifies those feelings.
If that's true, what happens to people who suddenly lose the ability to make certain expressions, such as people who get Botox injections to stop wrinkle lines around their eyes and foreheads?
Barnard College professor Joshua Davis looked into that when he conducted a study published this year with Columbia University neuroscientist Kevin Ochsner.
Dr. Davis' team showed positive and negative movie clips to women before and after they got either a Botox injection, which temporarily paralyzes the muscles around the injection sites, or an injection of an anti-wrinkle material known as hyaluronic acid, which plumps up the skin.
Botox didn't seem to affect the women's reactions to the "strong stimuli," which included a clip from "America's Funniest Home Videos" for the positive stimulus and a man eating a worm sausage on "Fear Factor" for the negative stimulus.
However, the women did react less strongly to the "mildly positive" documentary film clip they were shown. Women in the other group, which got the filler injections, reacted more strongly to the worm-sandwich clip after their treatment than before.
Dr. Davis' theory? In strong emotional situations, people may get signals from their autonomic nervous system (heart rate and sweating, for example), along with other brain signals -- all of which would make facial expressions less important. But in more ambiguous situations, the facial feedback might be more noticeable if it is absent.
As to the women who got the hyaluronic acid treatment around their upper lips, it might have made their faces more sensitive to the "eww" motion of the lip in response to the "Fear Factor" images.
In the end, he said, it may turn out that facial expressions are important both for how they make us feel and how other people can read what we are thinking.
"I find the argument compelling that our expressions evolved because they did reflect how we were feeling, so that made them a useful signal."
These websites can provide more information on today's topics.
• Paul Ekman's face research: www.paulekman.com
• David Matsumoto's face research: www.davidmatsumoto.com
• Moebius syndrome: www.moebiussyndrome.com
• Roberto Caldara's research: www.psy.gla.ac.uk/~roberto/
• Dacher Keltner's research: http://greatergood.berkeley.edu/dacherkeltner/
Mark Roth: firstname.lastname@example.org or 412-263-1130.