Prolonged middle childhood may be a recent evolutionary development:
The anatomy of middle childhood can be subtle. Adult teeth start growing in, allowing children to diversify their diet beyond the mashed potatoes and parentally dissected Salisbury steak stage. The growth of the skeleton, by contrast, slows from the vertiginous pace of early childhood, and though there is a mild growth spurt at age 6 or 7, as well as a bit of chubbying up during the so-called adiposity rebound of middle childhood, much of the remaining skeletal growth awaits the superspurt of puberty.
“Adulthood is defined by being skeletally as well as sexually mature,” said Jennifer Thompson of the University of Nevada, Las Vegas. “A girl may have her first period at 11 or 12, but her pelvis doesn’t finish growing until about the age of 18.”
The 18-year time frame of human juvenility far exceeds that seen in any other great ape, Dr. Thompson said. Chimpanzees, for example, are fully formed by age 12. With her colleague Andrew J. Nelson of the University of Western Ontario, Dr. Thompson analyzed fossil specimens from Neanderthals, Homo erectus and other early hominids, and concluded that their growth pattern was more like that of a chimpanzee than a modern human: By age 12 or 14, they had reached adult size.
Life for Neanderthals was nasty and short, Dr. Thompson said, and Neanderthal children had to get big fast, which is why they hurtled through adolescence at the equivalent of today’s chapter-book age. Our extreme form of dilated childhood didn’t appear until the advent of modern Homo sapiens roughly 150,000 years ago, Dr. Thompson said, when adults began living long enough to ease pressure on the young to hurry up and breed.
And what an essential luxury item middle childhood has proved to be. “It’s consistent across societies,” Benjamin Campbell, an anthropologist at the University of Wisconsin in Milwaukee said. “In middle childhood, kids start making sense.”
Parental expectations rise accordingly. “Kids can do something now,” said Dr. Campbell, who edited the special issue. “They can do tasks. They have economic value.”
Boys are given goats to herd and messages to deliver. They hunt and fish. Girls weave, haul water, grind corn, chop firewood, serve as part-time mothers to their younger siblings; a serious share of baby care in the world is performed by girls not yet in their teens.
Workloads and expectations vary substantially from one culture to the next. Karen Kramer and Russell Greaves of Harvard compared the average number of hours that girls in 16 different traditional cultures devoted each day to “subsistence” tasks apart from child care. Girls of the Ariaal pastoralists in northern Kenya worked the hardest, putting in 9.6 hours daily. Agriculturalist girls in Nepal worked 7.5 hours a day.
Then you come to the more laid-back lives of the foragers. The researchers focused on the Pumé, a foraging group in west-central Venezuela, where preadolescent girls do almost nothing. They forage less than an hour a day, significantly less than their brothers, and are very inefficient in what little they do. They prefer hanging out at the campsite. “Pumé girls spend their time socializing, talking and laughing with their friends, beading and resting,” Dr. Kramer said.
But most cultures mark the beginning of middle childhood with some new responsibility. Kwoma children of Papua New Guinea are given their own garden plots to cultivate. Berber girls of northern Africa vie to prove their worth by preparing entire family meals unassisted.
In the Ituri forest of Central Africa, Mbuti boys strive to kill their first “real animal,” for which they will be honored through ritualized facial scarring. And in the United States, children enter elementary school, for which they will be honored through ritualized gold starring.
In middle childhood, the brain is at its peak for learning, organized enough to attempt mastery yet still fluid, elastic, neuronally gymnastic. Children have lost the clumsiness of toddlerhood and can become physically gymnastic, too, and start practicing their fine motor skills. And because they are still smaller than adults, they can grow adept at a skill like, say, spear-tossing, without fear of threatening the resident men.
Middle childhood is the time to make sense and make friends. “This is the period when kids move out of the family context and into the neighborhood context,” Dr. Campbell said.
The all-important theory of mind arises: the awareness that other people have minds, plans and desires of their own. Children become obsessed with social groups and divide along gender lines, girls playing with girls, boys with boys. They have an avid appetite for learning the local social rules, whether of games, slang, style or behavior. They are keenly attuned to questions of fairness and justice and instantly notice those grabbing more than their share.
The mental and kinesthetic pliancy of middle childhood can be traced at least in part to adrenarche, researchers said, when signals from the pea-size pituitary at the base of the brain prod the adrenal glands to unleash their hormonal largess. Adrenal hormones like DHEA are potent antioxidants and neuroprotectants, Dr. Campbell said, and may well be critical to keeping neurons and their dendritic connections youthfully spry.
Evidence also suggests that the adrenal hormones divert glucose in the brain to foster the maturation of the insula and anterior cingulate cortex, brain regions vital to interpreting social and emotional cues.
In middle childhood, the brain is open for suggestions. What do I need to know? What do I want to know?