Presentation on theme: "Lecture 4: The Solow Growth Model"— Presentation transcript:
1Lecture 4: The Solow Growth Model L11200 Introduction to Macroeconomics 2009/10Lecture 4: The Solow Growth ModelReading: Barro Ch.3 : p52-672 February 2010
2Introduction Last time: first lecture on economic growth Considered data on cross-country growth ratesBegan model of economic growthThis time: expand the model of economic growthDevelop the ‘Solow Growth Model’Aim: to understand what determines economic growth and explain cross-county growth rates
3Where did we get to last week? Setup a production function with attributes of diminishing marginal product, constant returns to scale:Showed that with this production function and fixed A, growth in per capita output only possibly by increasing capital per worker:
4Next stepsGrowth in per capita output depends on growth in capital per worker, given by:What determines the growth of the capital stock?What determines the growth rate of labour?Can then calculate growth in capital per worker
51. Growth of Capital Stock Growth of capital stock depends on:How much new capital is added ‘investment’,How much existing capital depreciates (wears out)Assume a fraction of the capital stock δ, depreciates each period and has to be replaced.So household income (after depreciation) is given by:Households save some fraction, s, which they invest in new capital.
61. Growth of Capital Stock So have equation for change in capital:Can convert this into the growth rate of capital stock by dividing both sides by KThis is the equation for growth of capital stock
72. Growth rate of labourCapital investment depends on how much people decide to saveLabour force growth depends on how much people decide to reproduce.Assume this is constant growth rate, ‘n’So
8Growth of capital per worker So now have:Can express in per worker terms by dividing through by , soFrom earlier, can now substitute:
9So growth rate of capital stock per worker depends on: Labour force growth, n: negativelyDepreciation, δ: negativelySaving rate, s: positiveAll of the above are fixed‘Average Product of Capital’ : what determines this?
10Average Product of Capital Marginal Product of capital is given by:e.g. if a 1 unit increase in K causes a 10 unit increase in Y, then MPk=10Average product of capital is simplye.g. 10 units of K produce 50 units of Y, so average product per unit is 5
11IntuitionThe more capital you add to production, the less each additional unit adds to outputSo as capital increases, average product decreases.This explains the final part of:
12There are two ‘forces’ on the growth rate of capital stock per worker: Saving raises capital stock per worker. But as the capital stock grows, the average product of capital falls. So a fixed s (e.g. 5%) translates to a lower growth rate of capital at higher levels of capitalDepreciation and population growth lower capital per workerSo there is a level of capital per worker at which these two forces are equal: an equilibrium
13Production and Investment Depreciation and labour force growth start at a level produce , save depreciates increases by net effect on new level falling , falls produce , save depreciates increases byPeriod 1Period 2This is lower than beforeSo net positive effect is smaller
14Intuition Can increase y by increasing K to a point: Depreciation and population growth lowers kAt high levels of K, the saved part of the marginal product of additional capital is only just enough to offset depreciation and population growthSo diminishing marginal product limits the impact of increasing K upon k, and hence upon y
15Explaining k*k* is the level of capital per worker at which the positive effect of new investment is exactly matched by the negative effect of δ and nWhen k reaches k* it stops at the equilibrium level of capital per worker.We call this the steady state level of k*
16Implications for y* From earlier: We now know what factors determine and so what determinesSo starting from 1 unit of capital, per capita output will grow until and then stop growing at the steady state
17Summary Developed a growth model Capital and labour produce outputThey exhibit diminishing marginal returns: so adding labour cannot increase per capita GDP and capital investment can only increase it to a point.Next lecture: more on what the model predicts for growth rates, and for the impact of changing s, δ and n.