In discussing gun control in “Gun Science” [Skeptic], Michael Shermer first cites a 1998 paper in the Journal of Trauma and Acute Care Surgery concluding that guns in the home are much more likely to be used in criminal assaults or homicides than for self-defense. Oddly, that study accounted only for cases where criminals were killed or wounded and not the more typical scenario in which an attacker is scared away. Cases where the attacker is killed or wounded account for well less than 1 percent.

Shermer ignores the 2004 National Academy of Sciences report that rejected the 1998 study and similar ones. As the NAS report noted, this type of public health research fails to account for the endogeneity problem—that it is especially people who feel threatened who tend to acquire guns. Fixing this reverses the claims.

John R. Lott, Jr.
Author of More Guns, Less Crime, third edition (University of Chicago Press, 2010)

SHERMER REPLIES: The 1998 paper is one of several that determined how often guns in the home are used to injure or kill in self-defense, compared with how often they are involved in accidental shootings, criminal assaults and suicide attempts. Tragic shootings outnumber defensive ones by more than 40 to one. In addition to cases in which a homeowner chased a bad guy away, the 1998 study did not count how often guns in the home were used to threaten or to intimidate a family member, a spouse, a girlfriend or a neighbor.

The self-defense figures Lott cites were derived by extrapolating low-frequency responses to public opinion polls to the entire U.S. population and are thus wildly inflated. Police reports suggest the use of guns in self-defense is much less common. An audit of Atlanta Police Department reports of 198 home-invasion crimes identified three cases in which victims successfully defended themselves. Intruders got to the homeowner's gun twice as often.

As for the NAS report: when the committee opined about case-control research, it was criticizing an analytical method most, if not all, of its members had never employed. The endogeneity issue is a case in point. The committee speculated that any statistical association between guns in the home and violent death may exist because people acquire firearms in response to specific or perceived threats or because gun owners may be more or less violence-prone.

But Arthur Kellermann, the lead author of the 1998 study that had been referred to in the committee's report, has militated against that possibility*. For instance, in a 1993 case-control study, he questioned households about a wide range of risk factors for violence and took any differences into account through logistic regression. In addition, every household was matched with a control in the same neighborhood, which ensured similar socioeconomic status and exposure to crime.

Interestingly, the study found that a household's risk of homicide from an intruder was neither higher nor lower if a gun was kept there but that the risk of homicide from a family member or an intimate acquaintance was much higher.


In discussing potential meteor strikes in “Preventing the Next Chelyabinsk” [Advances], John Matson writes, “Fortunately, impacts on the scale of Chelyabinsk occur only once a century, so perhaps humankind will have figured out even better techniques by then,” thus falsely supporting the common misunderstanding that if something happens once a century, it won't happen again for 100 years. That would require collective memory among meteors.

As a statistician, I can offer a calculation: if we assume that meteor strikes are random and independent and follow a Poisson distribution with a mean of one per 100 years, then the probability of a strike in any given year is about 1 percent irrespective of recent strikes.

Mark Nicolich
West Amwell, N.J.


I enjoyed “The Strange Magic of Micro Movies,” by David Pogue [TechnoFiles], but Pogue discounts the human contribution to why “micro movies” such as GIFs and six-second Vine movies are so popular. The primary use of micro movies I've seen is to communicate emotion or a reaction to a situation, comment, picture or otherwise previously uploaded statement. One could communicate emotions through words or emoticons, but micro movies allow for a more profound message.

Furthermore, while GIFs are used to communicate many emotions, they are typically made with the additional intention of making individuals laugh. I have read multiple theories that state laughter has allowed our species to form larger and more connected social networks. What if these micro movies are the next revolution in such communication?

Edwin E. Rice IV
Columbus, Ohio


According to “Human Hybrids,” by Michael F. Hammer, modern humans and extinct archaic human species such as Neandertals were able to create fertile offspring. Doesn't this make modern humans and Neandertals part of one species?

Chantelle Tait
Clifton, Va.

Hammer notes the open question as to how Homo sapiens replaced Neandertals and shows that it is likely that some contemporary non-Africans received an antiviral stretch of chromosome (STAT2) from H. sapiens interbreeding with Neandertals. Is it possible that modern humans wiped out Neandertals by bringing viral disease with them (such as when Europeans “conquered” the New World) and that the reason why modern humans carry STAT2 is that only those Neandertals that had it survived long enough to mingle significantly with the virus-laden newcomers?

Doug McAfee
Bothell, Wash.

HAMMER REPLIES: Regarding Tait's question, although some paleoanthropologists believe Neandertals should be classified as a separate species of Homo, many still regard them as a subspecies of Homo sapiens. Opinions on this correlate with views on whether anatomically modern humans (AMH) originated via Replacement or via Assimilation and Hybridization. Many other mammals that diverged as recently as Neandertals and AMH are considered distinct species but can interbreed and produce fertile offspring.

While McAfee's idea is interesting, in all likelihood, AMH from Africa were more vulnerable to the novel environmental pathogens they encountered as they moved into Europe than Neandertals were to pathogens from the African migrants. In contrast to the Europeans who came to invade the New World, early AMH from Africa probably carried comparatively few pathogens because they would have lived in small hunter-gatherer groups with far less density than agricultural populations that formed tens of thousands of years later and with none of their exposure to pathogens from domesticated animals. People today probably retain STAT2 and other immune-related variants acquired from Neandertals because that DNA helped those early AMH from Africa survive new habitats.


My Boss the Robot,” by David Bourne, should have referred to Bourne as principal systems scientist at the Robotics Institute of Carnegie Mellon University.

*Correction (9/6/13): This sentence was edited after posting to fix a grammatical error.