The Future Has Thicker Tails than the Past: Model Error as Branching Counterfactuals

Posted: 24 May 2011  

Nassim Nicholas Taleb

NYU-Tandon School of Engineering

Date Written: May 23, 2011


Ex ante predicted outcomes should be interpreted as counterfactuals (potential histories), with errors as the spread between outcomes. But error rates have error rates. We reapply measurements of uncertainty about the estimation errors of the estimation errors of an estimation treated as branching counterfactuals. Such recursions of epistemic uncertainty have markedly different distributial properties from conventional sampling error, and lead to fatter tails in the projections than in past realizations. Counterfactuals of error rates always lead to fat tails, regardless of the probability distribution used. A mere .01% branching error rate about the STD (itself an error rate), and .01% branching error rate about that error rate, etc. (recursing all the way) results in explosive (and infinite) moments higher than 1. Missing any degree of regress leads to the underestimation of small probabilities and concave payoffs (a standard example of which is Fukushima). The paper states the conditions under which higher order rates of uncertainty (expressed in spreads of counterfactuals) alters the shapes the of final distribution and shows which a priori beliefs about conterfactuals are needed to accept the reliability of conventional probabilistic methods (thin tails or mildly fat tails).

Suggested Citation

Taleb, Nassim Nicholas, The Future Has Thicker Tails than the Past: Model Error as Branching Counterfactuals (May 23, 2011). NYU Poly Research Paper. Available at SSRN: or

Nassim Nicholas Taleb (Contact Author)

NYU-Tandon School of Engineering ( email )

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