Causa o correlazione?

The High Price of Motherhood – More Sex Is Safer Sex: The Unconventional Wisdom of Economics
Steven E. Landsburg

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Punti chiave: L’economista è un tale che dimestiera cerca di distinguere le cause dalle correlazioni. E’ abbastanza ridicolo lo spettacolo che offre chi gli ricorda che cause e correlazioni sono cose diverse – Nessuno ricorderebbe ad un biologo di lavare le provette – Segue esempio standard di quel che fa l’economista

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facing difficult trade-offs between family and career.

Note:IL DILEMMA STUDIATO DA AMALIA MILLER

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On average, a woman in her twenties will increase her lifetime earnings by 10 percent if she delays the birth of her first child by a year…For college-educated women, the effects are even bigger…

Note:RITARDO MATERNITÀ E REDDITO

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A woman who gives birth at 24 might be a different sort of person from a woman who gives birth at 25… Maybe the 24-year-old is less ambitious…

Note:PRIMA DIFFICOLTÀ

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Instead of comparing random 24-year-old mothers with random 25-year-old mothers, she effectively compared 24-year-old mothers with 25-year-old mothers who had miscarried at 24.

Note:TRUCCO: COMPARARE SOLO CHI HA AVUTO UN FIGLIO A 25 MA UN ABORTO SPONTANEO A 24

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But the comparison is still imperfect. Maybe miscarriages and low wages have a common cause—poor health, for example.

Note:SECONDA DIFFICOLTÀ

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Let’s compare 25-year-old mothers with those 24-year-old mothers who conceived while using birth control. Now you’ve got two groups of women, none of whom wanted to be pregnant at 24.

Note:ALTERNATIVA: CONFRONTARE SOLO LA GRAVIDANZA NON VOLUTA A 24 CON LA GRAVIDANZA A 25

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Again, the experiment is imperfect. Getting pregnant while on birth control might be a symptom of carelessness, and carelessness can be a liability in the workplace.

Note:TERZA DIFFICOLTÀ

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a bunch of women who all report that they’d been trying to get pregnant since they were 23. Some succeeded at 24; others at 25.

Note:CONFRONTARE SOLO DONNE CHE CERCAVANO UN FIGLIO A 23 ANNI

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None of these experiments—the miscarriage experiment, the birth-control experiment, and the “trying to get pregnant” experiment—is perfect, but all three point to the same conclusion.

Note:CONCLUSIONI UNIFORMI

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In this case, the result is that early motherhood is not only correlated with low wages; it actually causes them. That’s largely what good empirical economics is about—finding thoughtful and creative ways to distinguish between correlation and causation.

Note:ESSENZA DELLA BUONA ECONOMISTA

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Reminding an economist that correlation does not imply causation is like reminding a chemist to be sure his test tubes are clean.

ECONOMISTI E BIOLOGI