Letters

Lead or poverty’s later toll

Most toxic materials have the most deleterious effects at the earliest exposure ages, so I was puzzled by the study outcome in “School-age lead exposures may do more harm than earlier exposures” (SN: 6/6/09, p. 13). Did the study control for social and financial background? It would make sense for effects of background to be greater at age 6 than age 1. Tom DuBois, strong>Glens Falls, N.Y.

Richard Hornung of Cincinnati Children’s Hospital Medical Center says the study looked at socioeconomic status indirectly through maternal IQ and a standard measure of the home environment. He also notes that each of the study populations was relatively homogeneous, made up almost exclusively of inner city children, further reducing the chance that the findings were a result of differences in social and financial factors rather than lead exposures.em>—Janet Raloff

Multiversal logic

Dealing with the question of multiverses requires coping with logic as well as with infinity (“Success in coping with infinity could strengthen case for multiple universes,” SN: 6/6/09, p. 26). Alex Vilenkin’s argument “is based on the belief that people aren’t special” and thereby is circular. “Humans would most likely live in an average bubble” only if A) the distribution of bubbles under consideration resembles a normal distribution (such that near-average examples are more likely than far from average ones) and B) the sample of studied universes is randomly selected and large enough to be statistically representative. B is certainly untrue; A might not be true. Similarly, an infinite universe alone is not sufficient to produce Boltzmann brains — there must be an infinite supply of appropriate matter in it (all 
neutrons wouldn’t do, for example) with a sufficiently random distribution to cover all possible configurations. As the laws of physics and chemistry constrain the distribution of matter, matter distribution may not be random enough to include Boltzmann brains. Conversely, the existence of humans in no way disproves the existence of Boltzmann brains. Just because the highly nonrandom sample of the brains that we know of are all in animals doesn’t tell us what the average randomly sampled brain in the universe is like. These difficulties go both ways — without a statistically significant random sample of universes, we cannot tell scientifically what is normal. David Campbell, strong>Tuscaloosa, Ala.