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That exercise is meant to facilitate comparisons such as: did arrests of 20-year-olds in New York in 1992 diverge from those of 18-year olds in the same state and year?This automatically takes account of anything going on in the Empire state that year (such as a crack epidemic) that would have affected 18-year-olds and 20-year-olds alike.Even if abortion cuts crime, it is still immoral, they fulminate.
Mr Levitt says his case is based on a “collage of evidence”, of which the flawed test is one small piece.
He is, in particular, sceptical that crack undermines his thesis: it varied more by age group than by state, he says, hitting 17-year-olds in all states harder than 25-year-olds in any state.
That claim—first demonstrated by John Donohue, of Yale Law School, and Steven Levitt, of the University of Chicago, in an academic article in 2001*—is the kind of provocative and surprising conclusion that has made Mr Levitt's book, “Freakonomics”, such a runaway success this year.
Unwanted children, the story goes, are more likely to become criminals in later life.
The principal difference between the two age groups is that one was born after the Supreme Court legalised abortion and the other before. But Messrs Foote and Goetz have inspected the authors' computer code and found the controls missing.
In other words, Messrs Donohue and Levitt did not run the test they thought they had—an “inadvertent but serious computer programming error”, according to Messrs Foote and Goetz Fixing that error reduces the effect of abortion on arrests by about half, using the original data, and two-thirds using updated numbers. In their flawed test, Messrs Donohue and Levitt seek to explain arrest totals (eg, the 465 Alabamans of 18 years of age arrested for violent crime in 1989), not arrest rates per head (ie, 6.6 arrests per 100,000).
They offer the crack epidemic, which rose and receded at different times in different places, as an example.
Messrs Donohue and Levitt claim to control for such effects in the final test of their paper.
The birth rate rose from 1.9 to 3.7 children per woman in the space of a year.
A forthcoming study by Cristian Pop-Eleches, of Columbia University in New York, explores how these extra 1.8 children fared in later life.