When do human beings start to dream?

—William Keith, Houston, Tex.

Paul Li, lecturer of cognitive science at the University of California, Berkeley, replies:

PINPOINTING WHEN humans begin to dream remains an elusive challenge, although scientists have some ideas. There are researchers who argue that dreams originate as early as in the mother’s womb, whereas others posit that they first occur when a child’s brain becomes more developed, around five to seven years old.

Self-reports of dreams provide the only reliable evidence that a person can dream. Unfortunately, it is impossible to ask a newborn infant or a fetus whether it had a dream last night. Instead scientists can gather clues about when we begin to dream by monitoring certain physiological markers while a person is asleep, such as brain waves, muscle tension and eye movements.

One stage of sleep, in particular, often indicates when a person is dreaming. This stage, called rapid eye movement, or REM sleep, typically occupies about 20 percent of an adult’s night sleep. Newborn babies may spend more than 80 percent of their total sleep time in REM.

Fetuses also experience REM sleep. Studies using ultrasound have shown that fetuses exhibit REM sleep as early as the 23rd week of gestation.

Although scientists can detect REM activity in fetuses, they cannot know for certain whether this physiological activity, specifically eye movements, indicates that the fetus is dreaming. This inability to determine what is happening is because humans do not necessarily always dream during REM sleep, and humans can dream outside of this sleep stage.

But even if we could assume, for a moment, that fetuses dream, what would they imagine in their sleep? And how much would their dreams differ from those children and adults have? These questions are certainly worth sleeping on.

Do genes make people evil?

—Robert Schreib, Jr., Toms River, N.J.

Daniel Lametti, a neuroscientist at McGill University, responds:

THE MONTREAL apartment where I live is rife with evildoers—well, to be precise, there is at least one. A couple of weeks ago my newspaper, routinely delivered at 5 a.m. to my building’s lobby, disappeared before I could scurry out of bed to collect it. To thwart the criminal, I asked my deliveryman to hurl the paper onto my third-floor balcony (thankfully, he has a good arm).

Admittedly, newspaper theft ranks low on the scale of evil acts. Still, I wouldn’t steal a newspaper. I would like to think that under most circumstances I wouldn’t steal at all. But many people do, and many also commit crimes that are much more sinister.

Scientists would like to know the root causes of evil behavior: Is it a product of our genes or environment? The answer appears to involve a combination of the two.

Since the 1960s psychologists have found that children who were abused and neglected are more likely to commit crimes later in life. Even so, researchers noted that most youngsters who are mistreated do not grow up to be criminals. Now our genes come into the picture.

A 2002 study found that a particular variation of a gene predicted antisocial behavior in men who were mistreated as children. The gene controls whether we produce an enzyme called monoamine oxidase A (MAOA), which at low levels has been linked to aggression in mice. The researchers found that boys who were neglected and who possessed a variation of the gene that produced low levels of MAOA were more likely to develop antisocial personality disorder, commit crimes and grow up to have a violent disposition. But those living in a similar environment who produced more of the enzyme rarely developed these problems.

Psychopaths are arguably the evilest of the evildoers. A study published in August 2010 looked at psychopathic tendencies in teenagers with low socioeconomic resources. The researchers found that adolescents who had a variation of another gene, which contributes to how quickly serotonin is recycled in the brain and which has been linked to hostile behavior in children, were more likely to exhibit signs of psychopathy.

These two recent findings provide strong evidence that evil behavior—mass murder, armed robbery, and perhaps even newspaper theft—might be caused by the right set of genes interacting with the wrong environment.