The article reports an interesting observation: "Still, the 500-mile boundary between Finland and this Russian republic marks one of the steepest standard-of-living gradients in the world: Finns are seven times richer than their neighbors across the border. 'The difference is even greater than between Mexico and the U.S.,' Knip tells me."

Then the article gets into the main point: "Soon, studies from around the world showed similarly surprising results. But it was germ-laden dirt that seemed to matter, not air pollution. The children of full-time farmers in rural Switzerland and Bavaria, for example, had far fewer allergies than their non-farming peers. And a study following more than 1,000 babies in Arizona showed that, unless parents also had asthma, living in houses with dogs reduced the chances of wheezing and allergies later in life. Researchers proposed that the more microbial agents that children are exposed to early in life, the less likely they are to develop allergies and autoimmune diseases later on. Studies also showed that baby mice kept in sterile environments were more likely to face autoimmune disease, seeming to back what came to be called the 'hygiene hypothesis.'"

I'm still puzzling how hygiene (in this sense) can be measured accurately over the course of a lifetime, as my mother is a farmer's daughter who grew up in the Great Plains during the Dust Bowl era, and thus was probably exposed to plenty of allergens, but she has long struggled with hay fever and other allergy symptoms. My wife grew up in even poorer circumstances early in the development of a developing country, and spent a few years of her childhood literally living on a dirt floor. But she has allergies now too. I wonder how well exposure to dirt or to germs can really be quantified at the individual level for reliable treatment/control studies of this issue, rather than this kind of cross-sectional population comparison. For example, do I really, as a parent, have any clear idea whether my children get more exposure to dirt or less exposure to dirt than the typical American child who is allowed to play outdoors and build sand castles and mud dams? I have no idea, and I'm not sure anyone has an idea.

See "Warning Signs in Experimental Design and Interpretation"

http://norvig.com/experiment-design.html

by Peter Norvig, LISP hacker and director of research at Google, and "Worms, Germs, and Dirt: What Can They Teach Us About Allergies and Autoimmune Diseases?"

http://www.sciencebasedmedicine.org/index.php/worms-germs-an...

by Harriet Hall, M.D. for more information about what we will need to find out to be sure how much this interesting hypothesis has to do with our commonplace diseases.

As the article also points out, "These findings don’t mean that people should eschew basic hygiene. Its benefits are clear: In the past 60 years or so, our overall life expectancy has continued to rise. The trick for scientists is to determine exactly which early life exposures to germs might matter and identify the biology behind their potentially protective effect."

And because I've seen recent threads here on Hacker News that include comments underestimating just how much longevity and general health have improved in the developed world, I'll share some links on those issues, some of which I learned about from other Hacker News participants and others of which I learned about from a demography of aging researcher. Girls born since 2000 in the developed world are more likely than not to reach the age of 100, with boys likely to enjoy lifespans almost as long. The article "The Biodemography of Human Ageing" by James Vaupel,

http://www.demographic-challenge.com/files/downloads/2eb51e2...

originally published in the journal Nature in 2010, is a good current reference on the subject. Vaupel is one of the leading scholars on the demography of aging and how to adjust for time trends in life expectancy. His striking finding is "Humans are living longer than ever before. In fact, newborn children in high-income countries can expect to live to more than 100 years. Starting in the mid-1800s, human longevity has increased dramatically and life expectancy is increasing by an average of six hours a day."

http://www.prb.org/Journalists/Webcasts/2010/humanlongevity....

A comparison of period life expectancy tables and cohort life expectancy tables for men and women in Britain

http://www.ons.gov.uk/ons/rel/lifetables/period-and-cohort-l...

helps make the picture more clear. ("Period life expectancy" is what is usually reported for a whole country. But cohort life expectancy provides a better estimate of future lifespans of young people today,

http://www.time.com/time/specials/packages/article/0,28804,1...

and is still steadily on the rise around the world.) Life expectancy at age 40, at age 60, and at even higher ages is still rising throughout the developed countries of the world.

http://www.scientificamerican.com/article.cfm?id=longevity-w...