Lecture 2: Static Portfolio Choices

Lecture 2: Static Portfolio Choices • We cover the following topics in this part • Choice I: Insurance • Optimal insurance with loading • Optimal coinsurance • Optimality of deductible insurance • Choice II: Optimal Investment Portfolio • Portfolio of single risky and risk-free assets • The effect of background risk • Portfolio of multiple risky assets and risk-free assets L2: Static Portfolio Choice

Means in Managing Risks • Insurance • Risky assets with a full insurance is like investing in a risk-free asset • Partial insurance is like a combo of risk free assets (full insurance) and risky assets • Diversified portfolio • Risk-free asset and single risky asset • Multiple risky assets • Risk-free asset and multiple risky assets L2: Static Portfolio Choice

Static Portfolio Choice I: Insurance • Maximize an agent’s utility when there is costly or costless hedging contract available • The case of actuarially fairly priced insurance • Assuming loss follows a distribution of x (where x>=0); premium = P; Indemnity schedule = I(x) • Insurer reimburses policyholder for the full value of any loss, I(x)=x • When the premium is actuarially fair, P=EI(x)=E(x) • Suppose the insured is risk averse, how much is he willing to pay for the insurance? • With insurance, his expected utility is u(E(x)) • Without insurance, his expected utility is E(u(x))=u(E(x)-П) • Insurance increases the certainty equivalent by П • As a result, risk-averse agents would take full insurance when insurance prices are actuarially fair • When the insurance has loading, optimal insurance amount is determined by the transaction cost and risk reduction. L2: Static Portfolio Choice

Insurance Loading • Suppose the chance of the ship being sunk is ½. • Insurer’s loading is 10% of the actual value of the policy • Suppose the amount of insurance purchased is I, P(I)=(I/2)+0.1(I/2)=0.55I • A square-root utility function • Questions • What is the expected wealth? • What is the expected utility? • What is maximum utility? L2: Static Portfolio Choice

Optimal Coinsurance • Definition: I(x)=βx • Insurance pricing rule: • P(β)=(1+λ)EI(x)= βP0, where P0 =(1+ λ)Ex • Random final wealth = w0- βP0-(1- β)x L2: Static Portfolio Choice

Mossin’s Theorem • Full insurance (β*=1) is optimal at an actuarially fair price, λ=0, while partial coverage (β*<1) is optimal if the premium includes a positive loading, λ>0. • An intuition is that full insurance may be still optimal if the degree of risk aversion of the policyholder is sufficiently high. • This is not correct given that risk aversion is a second-order phenomenon. For a very small level of risk, individual behavior toward risk approaches risk neutrality. For risk neutral policyholder, any saving in transaction cost is good. • Also may have corner solution of β*=0 when λ>0, occurring λ≥λ* where • Undiversified risk is an alternative form of transaction cost (page 52) L2: Static Portfolio Choice

Comparative Statics in Coinsurance Problem • Proposition 3.2: Consider two utility function u1 and u2 that are increasing and concave, and suppose that u1 is more risk averse than u2 in the sense of Arrow and Pratt (Equation 1.7, page 11). Then, β1*>β2* • Sounds natural • Proposition 3.3: An increase in initial wealth will (i) decrease the optimal rate of coinsurance β* if u exhibits decreasing absolute risk aversion (proof see page 54 and the next slide) (ii) increase the optimal rate of coinsurance β* if u exhibits increasing absolute risk aversion (iii) cause no change in optimal rate of coinsurance β* if u exhibits constant absolute risk aversion • Proposition 3.4: An increase in the premium loading factor λ will cause β* to (i) decrease if u exhibits constant or increasing absolute risk aversion (ii) either increase or decrease if u exhibits decreasing absolute risk aversion L2: Static Portfolio Choice

Proof of Proposition 3.3 • The sign of әβ*/ әw is same as әH’(β)/ әw L2: Static Portfolio Choice

Deductible Insurance • Deductible provide the best compromise between the willingness to cover the risk and the limitation of the insurance deadweight cost. • Suppose a risk-averse policyholder selects an insurance contract (P, I(.)) with P=(1+λ)EI(x) and I(x) nodecreasing and I(x)≥0 for all x. Then the optimal contract contains a straight deductbile D; that is I(x)=max(0, x-D) L2: Static Portfolio Choice

Static Portfolio Choice II: Diversification • Investors who consume their entire wealth at the end of the current period • The case containing a risk-free asset and a risky asset • The risk-free rate of return of the bond is r. the return of the stock is a random variable x • Initial wealth w0 • α is invested in stock • Ending Portfolio Value=(w0- α)(1+r)+ α(1+x)=w0(1+r)+a(x-r)=w+ αy L2: Static Portfolio Choice

Optimal Investment in Risky Assets Other results in proposition 4.1 should also follow. What can we learn from the above condition? L2: Static Portfolio Choice

Further Thoughts • As long as there is a positive excess return y, investors should invest in the stock market • Participation puzzle • Under constant relative risk aversion, the demand for stocks is proportional to wealth: a*=kw. More specifically, we have • Equity premium puzzle • Assuming a reasonable level of risk aversion lead to unreasonable shares of investment in common stocks • Using historical data, μ=6%, and σ=16%. If R=2, the investment in equity is 117%. Evan when R=10, equity investment would be 23%. (EGS page 66) • Mehra and Prescott (85) L2: Static Portfolio Choice

Background Risk One way to explain the surprisingly large demand for stocks in the theoretical model is to recognize that there are other sources of risk on final wealth than the riskiness of assets returns. We want to compare α** with α*, the demand for risky asset when there is no background risk. Assuming v(z)=Eu(z+ε), we have We just need to check if v is more concave than u, utility function corresponding to y without background risk. L2: Static Portfolio Choice

v and u • To show v is more concave than u, we need show L2: Static Portfolio Choice

Conditions regarding Background Risk Consider the following three statements: 1. any zero-mean background risk reduces the demand for other independent risk 2. for all z, -u’’’’(z)/u’’’(z)>=-u’’(z)/u’(z) 3. absolute risk aversion is decreasing and convex. Condition 2 is necessary for condition 1 under the assumption that u’’’ is positive. Condition 3 is sufficient for condition 1 and 2. Power utility function satisfies (3). L2: Static Portfolio Choice

Portfolio of Risky Assets • Two assets following the same distributions of x1 and x2 that are independently and identically distributed • Perform expected utility maximization • If two assets are i.i.d., holding a balanced portfolio is optimal • Home bias L2: Static Portfolio Choice

Diversification in Mean-Variance Model • There are n risky assets, indexed by i = 1, 2, …, n • The return of asset i is denoted by xi, whose mean is µi and covariance between returns of assets I and j is σij • Risk free asset whose return r= x0 L2: Static Portfolio Choice

Diversification in Expected-Utility Model L2: Static Portfolio Choice

Interpretation • Investment Implication: all investors, whatever their attitude to risk, should purchase the same portfolio of risky assets. • Two-fund Separation L2: Static Portfolio Choice

Technical Note on Comparative Statics • Comparative statics: how the equilibrium condition changes when an exogenous variable changes L2: Static Portfolio Choice

Technical Note on Differentiation • Leibnitz’s Rule Example: Yu, Lin, Oppenheimer and Chen 2007, on setting up a model Other examples: comparative statics of option price L2: Static Portfolio Choice

Example L2: Static Portfolio Choice

Exercises • EGS, 3.2; 3.3; 4.2 • Set up a model for one of your ongoing projects • Provide a non-technical (i.e., no reference please) introduction of the paper • Identify the primary tradeoff in your story • Setup the model L2: Static Portfolio Choice

Lecture 2: Static Portfolio Choices