BMJ  2005;331:1023-1024 (29 October), doi:10.1136/bmj.331.7523.1023-b

Letter

Left handedness and breast cancer

Correlation is spurious

The first 100% of the full text of this article appears below.

EDITOR—Ramadhani et al present the results of the prospective study analysing the contribution of innate hand preference, among several other factors, to the development of breast cancer.¹ They conclude that innate left handedness and the development of premenopausal breast cancer are associated. The mechanism by which this occurs has not been established, although the authors speculate that the origin of the association may lie in intrauterine exposure to steroid hormones. Rather than being used as evidence for causality, the authors should consider whether the link between both handedness and the incidence of breast cancer and a common third variable (exposure to sex hormones in the womb) indicates that the apparent association is nothing more than a spurious correlation.² Until this has been addressed and controlled for, their conclusion that left handedness is related to increased risk of breast cancer is not valid.

University College Hospital, London NW1 2BU drbloor@tiscali.co.uk

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Competing interests: None declared.

1. Ramadhani MK, Elias SG, van Noord PAH, Grobbee DE, Peeters PHM, Uiterwaal CSPM. Innate left handedness and risk of breast cancer: case-cohort study. BMJ 2005;331: 882-3. (15 October.)[Free Full Text]
2. Stigler SM. Correlation and causation: a comment. Perspect Biol Med 2005;48: S88-S94.[Medline]

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Innate left handedness and risk of breast cancer: case-cohort study

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