Figure 2.

Changes in telomere length (TL) as a function of years since commencement of smoking predicted by three alternative models to explain the observed relationship between smoking and telomere length: (a) causation hypothesis, (b) selective adoption hypothesis and (c) mixed hypothesis (selective adoption + causation). We assume that smoking starts at t₀ for smokers and continues thereafter, whereas non-smokers never smoke. The solid (red) line represents the change in telomere length for smokers and the dotted (blue) line for age-matched non-smokers. The dashed lines at t₁ and t₂ represent two telomere measurements made at different time points after the commencement of smoking; in a longitudinal study of TL these could represent baseline and follow-up measurements, respectively. c_s and c_n are TL at the time of commencement of smoking for smokers and non-smokers, respectively; m_s and m_n are the slopes of the lines describing how TL changes with time for smokers and non-smokers, respectively. See text for further details. (Online version in colour.)