The middle child, I think, is now an “endangered species”. According to a study, in 1976,65 percent of mothers between aged 40 and 44 had three or more children. Today, nearly two-thirds of women with children have only one or two. Middle children will soon be the tiniest.

As a middle child, I am disappointed at the potential disappearance of middle children. I’m the middle of three—two boys, one girl—so I’m what’s sometimes regarded as a “classic middle child”.

Being a middle child is not something you are eager for. As one middle child said to me, “There is a thing called middle-child syndrome (综合征). I certainly was always aware that the middle was not a position to be admired, even as I came to see typical middle-child features in myself. Middle children are natural mediators (和事佬); I avoid disagreements and habitually act as the family peace-maker. Middle children tend to be private but also long for love; I keep to myself but do not exactly hate attention.”

According to studies, middles traditionally receive less economic and emotional support from their parents. They also typically have less close relationships with their mothers and fathers compared with other brothers or sisters, so they tend to have more friends to fill the feeling gap.

In a study conducted by the City College of New York in which participants were asked to choose words they associate with the first, last, and middle kids, positive words such as caring and ambitious were cited concerning all three birth orders. Only middles, however, were described with such negative terms as overlooked and confused. More importantly, middles may be many things and they were the only birth order with which no one connected the term “overindulged”. It is true: their parents couldn’t allow them to do or have whatever they want, especially when they are enough old, and they don’t ask for everything either even if they are eager to own it occasionally.

Laughing together is an important way for people to connect and bond. And though the causes of laughter can vary widely across individuals and groups, the sound of a laugh is usually recognizable between people belonging to different cultures.

But what about animals? Do they “laugh”? And are the causes of animal and human laughter alike? In humans, people may laugh when they hear a joke, or when they see something that they think is funny, though it’s unknown if animals’ intelligence includes what humans would call a sense of humor.

However, many animals produce sounds during play that are unique to that pleasant social interaction. Researchers consider such vocalizations to be similar to human laughter. Recently, scientists investigated play vocalization to see how common it was among animals. The team identified 65 species that “laughed” while playing — most were mammals (哺乳动物), but a few bird species demonstrated playful laughter too. Reports of playful laughter were notably absent in studies describing fish, perhaps because there is some question as to whether or not play exists at all in that animal group. This new study could help scientists to analyze the origins of human laughter.

But how can we identify play? Unlike fighting, play is usually repetitive and happens independently of other social behaviors, said lead study author Sasha Winkler, a doctor of biological anthropology at the University of California. When it comes to identifying it, “you know it when you see it,” Winkler told Live Science. One sign is that primates — our closest relatives — have a “play face” that is similar to the expressions of humans who are playing.

When Winkler previously worked with rhesus macaques, she had noticed that the monkeys panted (喘气) quietly while playing. Many other primates are also known to vocalize during play, she said, so a hypothesis (laughter in humans is thought to have originated during play) supported by the play-related panting laughter of many primate species was put forward.

People now still laugh during play, but we also integrate laughter into language and non-play behaviors, using laughter in diverse ways to express a range of emotions that may be positive or negative. Human laughter notably differs from other animals’ laughter in another important way: its volume. People broadcast their laughter loudly, often as a way of establishing inclusion. By comparison, when most animals laugh, the sound is very quiet — just loud enough to be heard by the laugher’s partner.

“It’s really fascinating that so many animals have a similar function of vocalization during play,” Winkler told Live Science. “But we do have these unique parts of human laughter that are also an important area for future study."

According to a new study， teens focus on rewards and have a hard time learning to avoid punishment or consider the consequences of alternative actions.

University College London researchers compared how teens and adults learn to make choices based on the available information. They tracked the way in which 18 volunteers aged 12-17 and 20 volunteers aged 18-32 completed tasks in which they had to choose between abstract symbols.

Each symbol was consistently associated with a fixed chance of a reward, punishment, or no outcome. As the trial progressed ， participants learned which symbols were likely to lead to each outcome and adjusted their choices accordingly. Teens and adults were equally good at learning to choose symbols associated with reward， but teens were less good at avoiding symbols associated with punishment. Adults also performed significantly better when they were told what would have happened if they had chosen the other symbol after each choice， while teens did not appear to take this information into account.

“From this experimental lab study we can draw conclusions about learning during the teen years. We find that teens and adults learn in different ways， something that might be relevant to education," said lead author Dr. Stefano Palminteri. " Unlike adults, teens are not so good at learning to adjust their choices to avoid punishment. This suggests that incentive systems based on reward rather than punishment may be more effective for this age group. Additionally, we found that teens did not learn from being shown what would have happened if they made alternative choices."

To interpret the results, the researchers developed computational models of learning and ran simulations （模拟）applying them to the results of the study. The first was a simple model, one that learned from rewards, and the second model added to this by also learning from the option that was not chosen. The third model was the most complete and took the full context into account, with equal weight given to punishment avoidance and reward seeking. For example, obtaining no outcome rather than losing a point is weighted equally to gaining a point rather than having no outcome.

Comparing the experimental data to the models, the team found that teens" behavior followed the simple reward-based model while adults" behavior matched the complete, contextual model. “Our study suggests that teens are more receptive to rewards than they are to punishments of equal value, ” said senior author Dr. Sarah-Jayne Blakemore. “As a result, it may be useful for parents and teachers to frame things in more positive terms.”