What are the three main theories that explain the shape of the yield curve?
CFA Level I covers three yield curve theories — pure expectations, liquidity preference, and market segmentation. I can recite the definitions, but I don't really understand the implications. When is the yield curve upward-sloping vs. flat vs. inverted under each theory?
The yield curve plots yields against maturities and its shape tells a story about market expectations, risk preferences, and supply-demand dynamics. Three theories explain why the curve takes a particular shape.
1. Pure Expectations Theory (Unbiased Expectations)
The yield curve reflects the market's consensus forecast of future short-term rates. Under this theory:
- Upward-sloping curve: The market expects short-term rates to RISE
- Flat curve: Rates expected to stay the same
- Inverted curve: Rates expected to FALL
There is NO risk premium — investors are indifferent between a 2-year bond and two consecutive 1-year bonds.
Example: If the 1-year spot rate is 4.0% and the 2-year spot rate is 4.5%, the implied 1-year forward rate one year from now is:
f(1,1) = [(1.045)^2 / 1.04] - 1 = 5.002%
Under pure expectations, the market genuinely believes the 1-year rate will be ~5.0% next year.
2. Liquidity Preference Theory
Investors prefer shorter maturities (more liquid, less risky) and demand a premium to hold longer-term bonds. This premium increases with maturity.
Implication: The curve has an upward bias. Even if the market expects flat rates, the curve will slope upward because of the liquidity premium.
- Upward-sloping curve: Could mean rates are expected to rise OR rates are expected to stay flat with a liquidity premium
- Flat curve: Rates expected to FALL (the expected decline offsets the liquidity premium)
- Inverted curve: Rates expected to fall SIGNIFICANTLY (overcoming the premium)
3. Market Segmentation Theory
Different investors operate in different maturity segments and don't cross over. The curve shape is determined by supply and demand within each segment independently.
- Banks prefer short-term instruments (matching short-term deposits)
- Pension funds prefer long-term bonds (matching long-term liabilities)
- Insurance companies prefer intermediate maturities
If pension funds flood the long end with demand, long-term yields fall, and the curve flattens — regardless of rate expectations.
Preferred Habitat Theory is a modification: investors have preferred segments but will cross over if compensated with sufficient yield premium.
Comparing the Theories:
| Feature | Pure Expectations | Liquidity Preference | Market Segmentation |
|---|---|---|---|
| Forward rates are unbiased forecasts? | Yes | No — biased upward | No relevance |
| Investors substitute across maturities? | Perfectly | Yes, with premium | No (or only with incentive) |
| Inverted curve means? | Rates will fall | Rates will fall sharply | Short-end supply/demand imbalance |
Exam Tip: If the question asks which theory implies forward rates are unbiased predictors, the answer is pure expectations. If it asks which explains the typical upward slope, it's liquidity preference.
For more yield curve analysis, check our CFA Level I course.
Master Level I with our CFA Course
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