In a standard Dan Brown 24-hour time limit, Langdon with his beautiful side-kick races to find a weapon of mass destruction created by a mad scientist as a solution to over-population in the world that is threatening the human species with extinction. This weapon is created to cut down a major chunk of the human population. Here, we see dark and twisted reflections of the neo-Malthusian theory at work.
Now, I suspect that this population angst is not really central to the action in the book, and since I just bought the book I haven't found out yet for sure (you can offer a spoiler alert, if you want). What I did discover is that Dan Brown introduces the topic of over-population by referring on page 144 to "a prominent nineteenth century mathematician and demographist name Thomas Robert Malthus." Technically, Malthus did take his degree from Jesus College at Cambridge in mathematics, but his biographers (most notably William Petersen) have made it clear that he aspired to, and became, a parish priest in the Church of England. To call him a mathematician is a bit of stretch. Indeed, eventually (years after the publication of his Essay on Population) he became a Professor of Political Economy, not a Professor of Mathematics. But a demographist??? What is that about? A Google search returned a few on-line dictionaries that define a demographist as, duh, a demographer. But I could not find a single reference to Malthus that called him a demographist. This was new (as in novel) to me, but I guess that I can now add a new title to myself as a demographist. If Dan Brown says so, it must be.