Prospect Theory Flashcards
What is the traditional approach in choices under risk and uncertainty?
- Expected Utility Theory = EUT.
Behavioral finance is critical about descriptive value of expected utility theory.
Prospect theory is a positive theory about choices under risk and uncertainty.
What is prospect theory and who developed it?
Kahnemand and Tversky developed prospect theory, based on simple choice experiment to identify how people make choices in the face of risk.
They use experimental evidence to support their theory. 2 important elements:
1) Value function
2) Probabilitity weighting
What is the value function?
- People’s valuations depend on gains and losses relative to a reference point. The reference point is often the status quo.
This is in contrast with expected utility theory which presumes that we care about absolute levels of wealth, not changes relative to a reference point.
People treat losses differently compared to comparable gains.
- Loss aversion: People dislike losses more than they like comparable gains, losses loom larger than comparable gains.
What are the key features of the value function?
People sometimes exhibit risk aversion and sometimes risk seeking behaviour depending on the nature of the problem.
People’s valuations depend on gains and losses relative to a reference point, that reference point is often the status quo.
Loss aversion: people dislike losses more than they like comparable gains. Losses loom larger than comparable gains.
Diminishing marginal sensititivity implies risk aversion in the domain of gains and risk seeking in the domain of losses.
What is the common ratio effect?
Expected utility says that choices are invariant to common ratios. Violation of this is called the common ratio effect.
Kahneman & Tversky’s explanation:
- Underweighing of small probabilities
- Degree of relative over-weighitng increases the smaller the probabilities
What is subcertainty?
Choices involving certainty are relatively more attractive. This implies that the weighting function must be quite steep close to certainty.
What is prospect theory?
Prospect theory is a theory that describes decisions under risk and uncertainty.
The model is descriptive. It tries to model how people make choices in real life, rather than how they should take optimal decisions
Formally prospect theory consists of a specification of:
(1) Mental accounts
(2) A value function defined over losses and gains
(3) A probability weighting function
What is the 2 step process?
Prospect theory models choices as a 2 step process:
1) Editing phase:
- Prospects are transformed into simpler representations
- Heuristic rules and operations are appliced to organize, reforumulate and narrow down the options on the table
- Operations include coding, segregation, cancellation, simplification and the detection of dominance
2) Evaluation process in phase 2: gamble with highest value is chosen.
What is coding
Gambles / prospects are coded in terms of losses and gains relative to a reference point.
What is combination?
Prospects can sometimes be simplified by combining the probabilities associated with identical outcomes.
Q= (100,0.3; 100,0.3: 0,0.4) –> q’ = (100, 0.6; 0, 0,4)
What is segregation?
Some prospects contain a riskless component that can be segregated from the risk component. So you get a gamble and a riskless component.
What is cancellation?
When different prospects share identical components, these components may be discarded or ignored
What is simplification?
Prospect might be simplified by rounding either outcomes or probabilities
Prospect q=(99, 0.51: 0, 0.49) is likely to be coded as an even chance to win 100: q’=(100, 0,5; 0, 0,5)
What is detection of dominance
Finding the dominating choice between 2 prospect: outcomes of first components could be the same but probabilities higher.
What is the evaluation stage?
Decision makers evaluate prospects and are assumed to choose the one with the highest value.
Overall value of an edited prospect is function v, expressed in v(x) and pii(p).
V(x) assigns to each outcome x a number which reflects the value of that outcome.
Pii(p) assigns to each probability p a decision weight which reflects the impact of p on the overall value of the prospect.
What is the value function?
Utility function defined over gains and losses, always measured relative to a reference point (eg. 0, buying price of a stock,…)
Characteristics:
1) Concave in the domain of gains = risk aversion
2) Convex in the domain of losses = risk seeking
3) Point of nondifferentiability at the origin
4) More steeply sloped to the left of the origing than to the right = loss aversion
5) Marginal value of losses and gains decreases = diminishing marginal sensitivity/ This implies risk aversion in the domain of gains and risk seeking in the domain of losses.
What is subaddivity?
Subadditivity implies the overweighing of small probabilities.
Subcertainty
A sure event is always overweighted, choices involving certainty are relatively more attractive.
Implication: values are less sensitive to variations in probabilities.
An increase in the probability of an event does have a lower impact on the overall value of a prospect than in expected utility theory.
What is subproportionality?
For a fixed ratio of probabilities, the ratio of the corresponding decision weights is closer to unity when te probabilities are low than when they are high.
What did Olsen examine?
1997: He examines the degree to which investment behaviors of protfolio managers is consistent with the implications of the value function of prospect theory.
3 anonymous mail surveys of professional investement managers (all CFA’s with major responsbilities of an institutional investment portfolio).
First surve: information about investment managers perception of and attitudes toward investment risk.
Second 2 studies were designed to study investment preferences.
What conclusion can we make from prospect theory?
Prospect theory is a descriptive model about how people make decisions under risk, based on experimental evidence.
What are the applications to finance of prospect theory?
Research connecting prospect theory and finance can be divided into 3 areas:
1) Cross section of average returns
2) Aggregate stock market
3) Trading of financial assets over time
Financial decisions closesly associated with risk, which makes prospect theory an obvious candidate.
Other applications: insurance
What does the cross section of average returns mean?
Why do some securities have higer average returns than others.
CAPM postulates that securities with higher betas should have average returns, but this prediction has not received a lot of empirical support: Fama & French
Question: Can prospect theory explain some of the cross section of average returns?
What dit Barberis & Huang analyze?
Barberis & Huang (2008) analysed an economy in which investors derive PT utility from the change in the values of their portfolios.
Prospect theory leads to a new prediction: a security’s skewness in the distribution of its returns will be priced (willingness to pay).
In particular: a positively skewed security, a security whose return distribution has a right tail that is longer than its left tail will be overpriced, relative to the price it would command in an economy with expected utility investors and it will earn a lower average return.
Intuition: by taking a significant position in a positively sked stock, investors give themselves a small chance of becoming wealthy should the stock turn out the be the next Google.
Under the probability weighting component of PT, investors overweight the tails of the distribution they are considering = small probabilities. They overweight the unlikely state of the world in which they make a lot of money by investing in the postively skewed stock.
This makes them overprice stock –> which leads to lower average returns.
