Thursday, March 21, 2013

I'm not sure I'm sold on Unified Growth Theory

Of course, I'm just getting into it, so take that with a grain of salt...

It's a beautiful idea that has an answer to the whole sweep of human history from Malthusian times to today.

I'm just not sure the answer makes sense.

First the demographic transition and take off is all driven by birth rates, which at a certain point start to fall while human capital and technological progress take off ("quality for quantity"). But what actually happens is a fall in death rates, not a fall in birth rates (which comes later - perhaps with a similar virtuous cycle to what UGT describes). But it seems to me the fall in death rates happens because of the technological growth, which is supposed to be caused by the initial population increase.

The other odd thing is that it seems to get the great divergence backwards, right? If population is driving technological change, why did the UK and the US take the lead? Why not India and China?

It's a very nice model. I think endogenizing population growth and introducing subsistence issues is the right way to go. I just think all the focus on the population as the instrument of change is probably wrong. I'm guesing technological shocks due to the scientific revolution instigated the change, and the population followed as UGT predicts it would. It seems odd to think that population took the lead. This would explain the fall in death rates before the fall in birth rates, and this would explain why the UK and the US and not India and China.


  1. I agree that technical advances came first, but population growth exhibits positive feedback with this, allowing for larger markets and greater specialization. Growth rather than size is more important as it indicates rising living standards due to technology lowering the cost of living. Only select technologies have a material effect, textiles originally. A higher initial death rate indicates a wealthier population to start with, more able to develop and expand technology.

  2. "from Malthusian times to today."

    Were there ever "Malthusian times"? I'm doubtful of that idea, it could be true, but it's not as clear cut as people claim. There's a lot of vagaries in it that hide things.

    "why did the UK and the US take the lead? Why not India and China?"

    This is a really big issue for studying the development, second only to the question "Why did the industrial revolution happen?"

    1. Gee, I took Malthusian times to be when Malthus was alive. Otherwise, what does it mean? B.C.F. (Before Chemical Fertilizer)?

  3. Min - Oh I just mean population gorwth kept in check by subsistence. Presumably, Current, it was locally transcended from time to time.

    re: "This is a really big issue for studying the development"

    Yep - which is why it seems like a big deal that UGT seems to produce the wrong answer (I would think... still familiarizing myself).

    1. Thanks, Daniel. :)

      BTW, my impression is that the decline in the death rate was the result of improved public health and sanitation (late 19th century). Much more related to science than technology.

    2. And technological change does play an important role in UGT - it's just driven by endogenous fertility behavior. I suspect that while fertility behavior may indeed be endogenous in the way UGT describes, (1.) death rates lead the demographic transition, not birth rates, (2.) that was due to a tech shock, and (3.) ergo, that tech shock was not caused by the endogenous feedback with fertility processes - even though those processes might perpetuate the tech growth.

      I brought up tech shocks in class as the only way to jump to take-off before a certain level of population density occurred. The professor agreed, but then said that that's the less interesting case (because it's exogenous and because the endogenous fertility is the whole feature of the model). That stuck with me - that was not a particularly satisfying claim to me. Granted, exogenous technological change isn't satisfactory either but it's probably what we should be paying attention to IMO.

      I have some concerns about models of endogenous technological change too, but that's the subject of another post...

  4. Everyone agrees that the birth rate is limited by subsistence. The question is if and when that limit is actually in force at various periods in history. Cultural forces must be in place to bring the birth rate that high. When I read about the history of England I'm doubtful that it ever happened there. Many people can be poor without the Malthusian limit being close.


All anonymous comments will be deleted. Consistent pseudonyms are fine.