When the residents of Buenos Aires want to change the pesos they do not trust into the dollars they do, they go to an office that acts as a front for thriving illegal exchange market.

As the couriers carry their bundles of pesos around Buenos Aires, they pass grand buildings like the Teatro Colon, an opera house that opened in 1908, and the Retiro railway station, completed in 1915. In the 43 years leading up to 1914, GDP had grown at an annual rate of 6%, the fastest recorded in the world. In 1914 half of Buenos Aires’s population was foreign-born. Its income per head was 92% of the average of 16 rich economies.

It never got better than this. Its income per head is now 43% of those same 16 rich economies; it trails Chile and Uruguay in its own backyard.

The country’s dramatic decline has long puzzled economists. “If a guy has been hit 700,000 shots it’s hard to work out which one of them killed him,” says Rafael di Tella. But three deep-lying explanations help to throw light on the country’s decline. Firstly, Argentina may have been rich 100 years ago but it was not modern. The second theory stresses the role of trade policy. Thirdly, when it needed to change, Argentina lacked the institutions to create successful policies.

Argentina was rich in 1914 because of commodities; its industrial base was only weakly developed. The landowners who made Argentina rich were not so bothered about educating it: cheap labor was what counted.

Without a good education system, Argentina struggled to create competitive industries. It had benefited from technology in its Belle Epoque period, but Argentina mainly consumed technology from abroad rather than inventing its own.

Argentina had become rich by making a triple bet on agriculture, open market and Britain, its biggest trading partner. If that bet turned sour, it would require a severe adjustment. The First World War delivered the initial blow to trade. Next came the Depression, which crushed the open trading system on which Argentina depended. Dependence on Britain, another country in decline, backfired( 失 败 ) as Argentina’s favored export market signed preferential deals with Commonwealth countries.

After the Second World War, when the rich world began its slow return to free trade with the negotiation of the General Agreement on Tariffs and Trade in 1947, Argentina had become a more closed economy. An institution to control foreign trade was created in 1946; the share of trade as a percentage of GDP continued to fall. High food prices meant big profits for farmers but empty stomachs for ordinary Argentines. Open borders increased farmers’ taking but sharpened competition from abroad for domestic industry. Heavy export taxes on crops allow the state to top up its decreasing foreign-exchange reserves; limits on wheat exports create surpluses(过剩) that drive down local prices. But they also dissuade farmers from planting more land, enabling other countries to steal market shares.

It is not hard to find evidence of the success of the “sharing economy”, in which people rent beds, cars and other underused assets directly from each other, or via the internet. One pointer is the large amount of demand and supply. Airbnb claims that 11m people have used its website to find a place to stay. Lyft, a company that matches people needing rides and drivers wanting a few dollars, has spread from San Francisco to 30-odd American cities. Another sign is the frothy values (泡沫价值) placed on sharing-economy companies: Airbnb is estimated to be worth $10 billion, more than hotel chains such as Hyatt and Wyndham, and Lyft recently raised $250m from venture capitalists. But perhaps the most flattering—and least welcome—indicator of the sharing economy’s rise is the energy being devoted by governments, courts and competitors to preventing it.

The main battlegrounds are the taxi and room-rental businesses. A court in Brussels has told Uber, another San Francisco ride-sharing and taxi-services startup, to stop operating in the city. Other cities have banned their services outright, or tried other ways of putting spokes in their wheels. Meanwhile the Hotel Association of New York has been lobbying for (游说) stricter enforcement of a rule that bans absent owners from letting their apartments for less than 30 days, which makes most of Airbnb’s listings there illegal.

The newcomers’ opponents, whether competitors, officials or worried citizens, complain that the likes of Airbnb and Lyft dodge (躲避) the rules and taxes that apply to conventional businesses. Regulations exist to keep hotel rooms clean and fire alarms in working order, to stop residential areas being filled with unlicensed hotels, and to see that drivers are insured, checked for criminality and tested on their knowledge of the streets. Cowboys such as Airbnb, Lyft and Uber, their critics claim, are a danger to an unsuspecting public.

The objectors have half a point. Taxes must be paid: a property-owner who rents a room should declare the income, just as a hotel should. Safety is also a concern: people want some assurance that once they bed down for the night or get into a stranger’s car they will not be attacked or robbed. Zoning (划分区域) and planning are also an issue: peace-loving citizens may well object if the house next door becomes a hotel.

Sharing-economy firms are trying to mitigate (缓和) these problems. They have tightened insurance cover for their drivers and have offered to collect hotel taxes. They have an interest in their participants’ good behavior: as hosts, guests, drivers and passengers all rate each other online, their need to protect their reputation helps to maintain standards and keep people honest. But if consumers want to go for the cheaper, less-regulated service, they should be allowed to do so.

The truth is that most of the rules that the sharing economy is breaking have little to do with protecting the public. The opposition to Lyft and Uber is coming not from customers but from taxi companies, which understand that GPS makes detailed knowledge of the streets redundant (多余的) and fear cheaper competition.

This all argues for adaptation, not prohibition. An unlikely pioneer is San Francisco. Lyft and Uber got going in the city partly because taxis were hard to find, but the authorities have tolerated them. San Francisco bans rentals of less than 30 days, but is considering allowing people to let their residence, provided they live there most of the time, register with the city and pay its 14% hotel tax.

A team of University of Wisconsin-Madison researchers has shown that chronic (长期的) stress of being poor, lacking care and physical abuse in early life may shrink the parts of a child' s developing brain responsible for memory, learning and processing emotion.

While early life stress already has been linked to anxiety, heart disease，cancer and a lack of educational and employment success,researchers have long been seeking to understand what part of the brain is affected by stress so as to help guide interventions(干预).

Seth Pollak, co-leader of the study and a UW professor of psychology,says the research focused on two brain areas—the hippocampus (HC:海马体) and amygdale (AG: 杏仁体).

For the study, Pollack’s team had 128 children around age 12 and divided them into four groups after interviews with the children and their caregivers，documenting behavioral problems and their accumulated life stress.

One group had experienced physical abuse,another group was ignored before being adopted from a foreign country, and a third group came from low-income families. The fourth group of children came from middle-class families and had not experienced any of the three types of chronic early stress.

Researchers did MRI scans (核磁共振扫描) of the children' s brains, focusing on the HC and AG. Then they painstakingly tracked those areas of the brain by hand on paper.

The hand measurements showed that children who had experienced poverty, neglect or physical abuse had a smaller AC and HC than the children from middle-class families.

"I think we added something solid to the literature," he said. "Our research may inform social policy and professional interventions to help the children who are suffering."