Tail dependence

Definition

Tail dependence in plain English

Tail dependence asks a scary question: when one thing has an *extreme* outcome, does the other tend to go extreme at the same moment? Two investments might drift apart casually day to day, yet still crash together in a crisis — that joint-disaster tendency is tail dependence, and it's exactly the risk that wipes out portfolios people thought were diversified. A famous trap is the Gaussian (bell-curve) copula, a tool once used to model how mortgages default together: it assumes that in the far extremes things become essentially independent, so it badly underestimated how many loans would fail *simultaneously* — a blind spot blamed for deepening the 2008 crisis. The lesson: normal-looking correlations can hide the fact that the worst events love to arrive hand in hand.

Learn these first

• PCA (Principal Component Analysis)
• Unbiased estimator

Related Quantitative Finance terms

• Alpha
• Autocorrelation
• Bayes' theorem
• Beta
• Copula
• Correlation
• Eigenvalue
• Generalized Autoregressive Conditional Heteroskedasticity (GARCH)
• Markov property
• Maximum likelihood