Final exams are around the comer - but that won’t stop some teenagers putting in the least effort. This may be because their brains aren’t developed enough to properly assess how high the stakes (利害关系) are, and adapt their behaviour accordingly.

Catherine Insel, at Harvard University, and her team asked adolescents between the ages of 13 and 20 to play a game while monitoring their brains. In some rounds of the game, participants could earn 20 cents fora correct response, while an incorrect one would cost them 10 cents. But in rounds with higher stakes, correct responses were worth a dollar, and wrong answers lost the participants 50 cents.

The team found that while the older volunteers performed better in the high stakes rounds, the younger ones didn’t - their performance didn’t change in line with whether the stakes were low or high. And the older the volunteers were, the more improved their performance was.

When the team looked at the brain activity of the volunteers, they found that their ability to improve their performance was linked to how developed their brains were. A region in the brain, which continues to develop until we are at least 25 years old, seemed to be particularly important. The findings explain why some teenagers are so unconcerned when it comes to hazardous behaviors, such as driving too fast, for instance, especially when one of their friends is nearby.

Insel thinks schools should reconsider the way they test performance in teenagers. “This study suggests it’s not a good idea to evaluate school performance in a single final exam”, she says. A better idea would be to use a variety of smaller tests, conducted throughout the year.

It’s not all bad news for teens. though. Teenagers put the same amount of effort into tasks that aren’t important, and start to prefer hobbies to school. It could be a good thing, allowing teenagers to learn complex social skills, for example.

A new study suggests that identical twins are not exactly the same genetically. Identical twins are two babies that come from the same fertilized egg (受精卵). Scientists in Iceland examined DNA from 387 pairs of identical twins, their parents, children, husbands or wives. “The examinations led the team to find early mutations (突变) that separate identical twins,” lead researcher and geneticist Kari Stefansson said. He is a professor at the University of Iceland and founder of the company deCODE genetics.

Mutations are small changes in DNA that can happen when a cell divides in an attempt to copy itself. These small changes can influence a person’s physical appearance or control a person’s ability to fight a disease. The newly-discovered mutations show that identical twins do have genetic differences, the researchers said. The results were recently published in Nature Genetics.

On average, identical twins have 5.2 of these early genetic differences, the researchers found. But about 15 percent of identical twin pairs have more than that. Some may have as many as 100 genetic differences, Stefansson said. These differences represent a small part of each twin’s genetic material. But they could influence why one twin is taller or why one is at greater risk of some cancers than the other.

In the past, many researchers believed physical differences seen in identical twins were related mostly to environmental influences, such as nutrition or lifestyle behaviors. Jan Dumanski is a geneticist at Sweden’s Uppsala University. He was not involved in the study. He praised the findings as a clear and important contribution to medical research. “It suggests we have to be very careful when we are using twins as a model for examining the influences of genetics or the environment,” Dumanski said.

A 2008 paper in The American Journal of Human Genetics found some genetic differences between identical twins. The new study, however, goes beyond earlier work by including the DNA of parents, children, husbands and wives of identical twins. Studying family members permitted the researchers to examine when genetic mutations happened in two different kinds of cells: those present in only one individual and those passed on to the person’s children.

Stefansson said his team found twins where a mutation is present in all cells of the body of one twin, but not in the other twin at all. “However, sometimes the second twin may show the mutation in some cells, but not all cells,” he added. The researchers said they also found mutations that came about before the developing embryo (胚胎) split in two.

Nancy Segal is a psychologist who studies twins at California State University, Fullerton. She was not involved in the study. But she called the result “heroic and really significant”. Segal added that the research is likely to persuade more scientists to rethink the influences of genetics and environment on twins. “Twins are very alike, but it is not a perfect similarity,” she said.

When you eagerly dig into a long-awaited dinner, it’s traditionally believed that signals from your stomach to your brain stop you eating so much. However, a research team recently discovered that it’s our sense of taste that immediately pulls us back from eating food overly on a hungry day. Stimulated by the perception of flavor, a set of brain cells become active to quickly curtail food intake.

Previous studies have suggested that the food taste may control how fast we eat, but it’s been impossible to study relevant brain activities during eating because brain cells that control this process are located deep in our brains, making them hard to access or record in an awake animal. New techniques developed by the team allowed for the first-ever imaging and recording of a brainstem structure critical for feeling full, called NTS, in an active mouse.

The new study found that when researchers put food directly into the mouse’s stomach, brain cells called PRLH were activated by signals from the gut (消化道), in line with traditional thinking. However, when they allowed the mice to eat the food as they normally would, those signals from the gut didn’t show up. Instead, the PRLH brain cells switched to a new activity pattern that was entirely controlled by signals from the mouth. “It’s astonishing that these cells were activated by the perception of taste,” said researchers. “It shows that there are other components of the appetite-control system that deserves our attention.”

The PRLH-activated slowdown also makes sense in terms of timing. The taste of food allows PRLH to switch their activity in seconds. In contrast, another group of brain cells, called CGC, takes several minutes to respond to signals from the gut. The good thing is that CGC can hold back hunger for a much longer time. These two sets of brain cells interact to work together: one uses taste to slow down eating, while the other signals that you are full.