From the Introduction: "Malthus began with two physiological assumptions: humans must have food, and the sex drive will always be a fundamental part of our make-up. (Both assumptions had been called into question, half-seriously, by Godwin.) His next assertions were less self-evident but crucial to the argument: an unchecked population grows at a 'geometric' rate, as in the series 1, 2, 4, 8, 16, and the means of subsistence can only be increased at an 'arithmetic' rate, as in 1, 2, 3, 4, 5. Because man's powers of reproduction so greatly exceed his powers of food production, population will always press against the available resources. Thus a substantial portion of society is condemned to live at the ragged edge of subsistence. Any significant rise in general living standards will trigger a period of earlier marriages and lower mortality, bringing faster growth of population than of food supplies. Per capita consumption, having risen temporarily above 'subsistence' level, will be forced back down to that level, or even below it. Almost as famous as this grim analysis, which prompted Thomas Carlyle to dub economics the 'dismal science', is the conceptual apparatus that supports it. Malthus argued that population was held within resource limits by two types of 'checks': positive ones, which raised the death rate, and preventative ones, which lowered the birth rate. The positive checks included hunger, disease, and war; the preventative checks, abortion, birth control, prostitution, postponement of marriage, and celibacy. All of these population retardants, without exception, led mankind into 'misery' or 'vice'. Thus commentators have mapped out four Malthusian quadrants of woe: positive of misery (disease; malnutrition) or of vice (the waging of war), and preventative checks of misery (the postponement of marriage; celibacy) or of vice (prostitution; birth control).
Let me first say, most emphatically, that Malthus was not wrong; anyone who believes that Malthus was wrong is either misguided, or simply restating something they heard another misguided person say. The fact of the matter is that Malthus has never been a popular figure (it's rumored that Charles Dickens based his character Ebenezer Scrooge on Malthus) and in today's extremely bi-partisan environment - it's a pretty safe bet to say that he would be sitting in the Republican aisle of Congress. Nevertheless, and all politics aside, much of what has been attributed to Malthus has been reverse-engineered to make him sound like a cold-hearted elitist prude, which he wasn't. I only recommend reading this book and making up your own mind.
Lastly, this is really one of my favorite polemics, so naturally I am biased; however, I can't help but see Malthus in many of my other favorite books: Jared Diamonds - Collapse: How Societies Choose to Fail or Succeed: Revised Edition, Garrett Hardin's - Living within Limits: Ecology, Economics, and Population Taboos, Nafeez Ahmed - A User's Guide to the Crisis of Civilisation: And How to Save it, or Chris Martenson's - The Crash Course: The Unsustainable Future Of Our Economy, Energy, And Environment. I think the ideas of 'The Tragedy of the Commons', 'The Tyranny of Small Decisions', and even the great big theory of 'Darwinian Evolution', all have their genesis in Thomas Malthus and An Essay on the Principle of Population. This is a great book - possibly required reading even - and at about 175 pages, including the Introduction, I think everyone might want to read it.