The American smoking rate is going down. The obesity rate is going up. And when it comes to women's life expectancy, those trends have just about canceled each other out.

A team of researchers with the National Bureau of Economic Research published data looking at obesity and smoking rates. The trends are, as shown in this graph below, not exactly subtle:

Both of those trends have an impact on longevity; these researchers wanted to tease out how each affected lifespan. Overall, the benefits of less smoking are outweighing the negatives of rising obesity - but only by a little bit, especially when it comes to women.

The combined effect of changes in smoking and obesity is expected to produce steady improvements in male life expectancy through 2040, with a total gain of 0.92 years by that date. On the other hand, women’s life expectancy is expected to be lower as a result of the combined changes through 2030. Thus, the pattern of reductions in the female advantage in life expectancy that has been evident since 1979 is expected to continue for another two decades, at least from these sources. By 2040, life expectancy is anticipated to be 0.26 years higher for females as a result of these combined behavioral changes.

That assumes that current trends hold, although there is some evidence that tobacco use is going up among young adults, which could change some of the calculus here.