Why is it thought that left-handers

live less long?

Chris McManusUniversity College London

Statistical Fallacies in Research & the MediaRoyal Statistical Society

1st June 2015

• And the other four reasons?

That can wait until later…

Stanley Coren and Diane Halpern, 1988, Nature, “Do right-handers live longer?”

“[a]bout 2 per cent more right-handers than left-handers survive at each age (p<.001) (see figure)”.

Is this difficult to interpret?

t-test, t=0.62, 1706 df, p=0.537Diff= 35 weeks

Nature, 16th June 1988

Several letters, but none on the statistics…

Nature, 18th April 1991

Nature, 18th April 1991

New England Journal of Medicine, April 4th, 1991

New England Journal of Medicine, Oct 3rd , 1991

Handedness evaluated in 1976 or 1978; followed up in 1988

Ken Rothman, Epidemiologist

The problem with the Halpern and Coren study.

• They knew the ages of– dead right-handers, and – dead left-handers

• But, they did not know the ages of– living right-handers and – living left-handers

• If living left-handers are also younger than living right-handers– Secular trend – But no effect on survival

• Halpern and Coren’s study was a ‘death cohort study’– Death cohorts always confound secular trends and age at death– ‘Birth cohort studies’ do not suffer from that problem.

Are there secular trends in handedness?

1900 1910 1920 1930 1940 1950 1960 1970

Year of Birth

0%

2%

4%

6%

8%

10%

12%

14%

16%

FemalesMales

Gilbert & Wysocki (1992) Neuropsychologia, 30: 601-8

Secular trends

1900 1910 1920 1930 1940 1950 1960 1970

Year of Birth

10%

2%

3%

4%

5%

6%

7%8%9%

20%

FemalesMales

Gilbert & Wysocki (1992) Neuropsychologia, 30: 601-8

Sex differences

Combining all the nineteenth century data

1760 1780 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 20001%

10%

2%

3%

4%

5%

6%

7%8%9%

20%

So is this problem only found in studies of handedness?

Err, No…!

Lancet, 1994

BMJ, 1996, Christmas edition

Finally, we committed a statistical howler in our Christmas issue. We published a paper that concluded from obituary data that doctors from the Indian subcontinent and anaesthetists died young. Chris McManus suggests that the same data might have shown that doctors who had Oasis CDs or who were called Tracy or Kevin also died young (p 1132). It's a problem of denominators. There were none in the paper, and the seemingly increased risk of doctors from the Indian subcontinent and anaesthetists of dying young is probably because the population from which they come is young. Many people wrote to point out the error. We are comforted by the Lancet having made the same mistake two years ago.

Finally, we committed a statistical howler in our Christmas issue. We published a paper that concluded from obituary data that doctors from the Indian subcontinent and anaesthetists died young. Chris McManus suggests that the same data might have shown that doctors who had Oasis CDs or who were called Tracy or Kevin also died young (p 1132). It's a problem of denominators. There were none in the paper, and the seemingly increased risk of doctors from the Indian subcontinent and anaesthetists of dying young is probably because the population from which they come is young. Many people wrote to point out the error. We are comforted by the Lancet having made the same mistake two years ago.

Finally, we committed a statistical howler in our Christmas issue. We published a paper that concluded from obituary data that doctors from the Indian subcontinent and anaesthetists died young. Chris McManus suggests that the same data might have shown that doctors who had Oasis CDs or who were called Tracy or Kevin also died young (p 1132). It's a problem of denominators. There were none in the paper, and the seemingly increased risk of doctors from the Indian subcontinent and anaesthetists of dying young is probably because the population from which they come is young. Many people wrote to point out the error. We are comforted by the Lancet having made the same mistake two years ago.

Finally, we committed a statistical howler in our Christmas issue. We published a paper that concluded from obituary data that doctors from the Indian subcontinent and anaesthetists died young. Chris McManus suggests that the same data might have shown that doctors who had Oasis CDs or who were called Tracy or Kevin also died young (p 1132). It's a problem of denominators. There were none in the paper, and the seemingly increased risk of doctors from the Indian subcontinent and anaesthetists of dying young is probably because the population from which they come is young. Many people wrote to point out the error. We are comforted by the Lancet having made the same mistake two years ago.

Finally, we committed a statistical howler in our Christmas issue. We published a paper that concluded from obituary data that doctors from the Indian subcontinent and anaesthetists died young. Chris McManus suggests that the same data might have shown that doctors who had Oasis CDs or who were called Tracy or Kevin also died young (p 1132). It's a problem of denominators. There were none in the paper, and the seemingly increased risk of doctors from the Indian subcontinent and anaesthetists of dying young is probably because the population from which they come is young. Many people wrote to point out the error. We are comforted by the Lancet having made the same mistake two years ago.

And yet still they keep coming…

This is clearly a modern myth (and there seems to be no sign of it before 1988). So why is it so prevalent and so resistant to evidence?

• #4: They’re more likely to go insane• #3: They’re screwed at school• #2: They’re more easily scared…• #1: Hating them is ingrained in our

culture

• So what were the other four reasons?