Yesterday, we learned that human brains shrink but chimpanzee brains do not. It's a curious finding that brain-scanning researchers explained to The Wall Street Journal as the price of a human's longer lifespan: we live to around 80 years or so and chimps can live to about half that. Therefore, the explanation goes, our relative older age accounts for that normal 15 percent brain shrinkage we experience.

The finding got us thinking about brain shrinkage--it can literally be when gray matter inside your head thins out. And even though it's described as a normal part of aging, there have been numerous studies linking brain shrinkage to things other than, say, stress and depression. Below are just a few of those news-reported items academics say also appear to hasten the process:

  • Chronic back pain may cause brain shrinkage by up to 11 percent. In 2004, Northwestern University researchers linked chronic back pain to a thinning of gray matter. In more specific terms than those with fragile backs would like to think about, the lead researcher explained in a press release at the time: "Loss in brain density is related to pain duration, indicating that 1.3 cubic centimeters of gray matter...are lost for every year of chronic pain."
  • Alcohol (but wine more so than beer) may cause brain shrinkage. In a 2008 study recounted in the UK Independent, researchers found that "the hippocampus, the part of the brain involved in memory, spatial tasks and many other functions, was more than 10 percent smaller in those whose tipple was wine than in those who favoured beer."
  • Internet addiction may cause brain shrinkage. In June, Scientific American reported on a recently published Chinese study that brain-scanned 18 college-aged internet addicts and found, in part, that "several small regions in online addicts' brains shrunk, in some cases as much as a 10 to 20 percent."
  • Sleeplessness may cause brain shrinkage. A 2010 study linked chronic insomniacs to the loss in brain density, and researchers found, as Discovery News wrote, "brain imaging has linked chronic insomnia to lower gray matter density in areas that regulate the brain's ability to make decisions and to rest." The caveat: "the researchers are not yet able to pin down whether sleeplessness precedes gray matter loss or the other way around."
  • A vegetable-heavy diet may cause brain shrinkage. That's a simplistic way of saying a diet with a lack of the vitamin B12. In a counter-intuitive finding in 2008, researchers found that the lack of a diet including the vitamin B12, found in meats and milk, isn't good for the brain. The study was described by The Sun as concluding: "those on a meat-free diet are six times more likely to suffer brain shrinkage as the most vitamin B12 is found in meats, liver, fish and milk."
  • Lack of sunlight may cause brain shrinkage. There's a big caveat to this 2007-reported study: it was only performed on mice. But, like a lot of other research, the findings were implied for humans. One of the Ohio State researchers described to the Associated Press: "If humans' brains similarly shrink without sunlight exposure, it could be physical evidence of depression some people complain about when daylight decreases."
  • Marijuana usage may cause brain shrinkage. It may be a delicate, controversial subject for medical marijuana proponents, but we'll just note a 2008 Reuters report describing a brain-scanning study that found "the hippocampus and amygdala were smaller in men who were heavy marijuana users compared to nonusers, the researchers said. The men had smoked at least five marijuana cigarettes daily for on average 20 years."
  • Religious affiliation  may cause brain shrinkage. When this bit of research surfaced, the ensuing headlines humored a few religious skeptics. But the finding is more nuanced than merely linking the faithful to shrinking brains. As described by the research magazine Miller-McCune in May, the study found:

This accelerated shrinkage was also found in people who reported a life-changing religious experience, as well as those with no religious affiliation.The reason, the researchers speculate, is the cumulative stress that comes with being a member of a religious minority.