Otago researchers find "dark shift" bias in self-reported skin colour

New University of Otago research has found that people over a range of different skin colours tend to overestimate the darkness of their natural, non-tanned skin.

The researchers identifying the pattern have called it “the dark shift” and believe it may have important implications for how well skin cancer risk messages are heeded.

The findings, based on a study involving nearly 300 University of Otago students, are published in the May edition of the American Association for Cancer Research journal Cancer Epidemiology, Biomarkers & Prevention.

Lead author Dr Tony Reeder, of the Cancer Society Social & Behavioural Research Unit at the University, says the pattern emerged after comparing the students’ self- reported “natural, non-tanned skin colour” to objective measurements taken with a special instrument called a spectrophotometer.

Among the findings were that 36 per cent of those reporting that they had ‘fair skin’ actually fell into the range for ‘very fair skin’, while just over 77 per cent of those believing they were of medium skin colour actually belonged to the ‘fair skin’ category.

Study participants who considered their natural skin colour to be “olive” also strongly overestimated, with 71 per cent falling into lighter categories. Of those who rated themselves as having dark skin, 58 per cent were classified as belonging to a lighter category.

Dr Reeder says that the study findings have the potential to impact significantly on health.

“The link between skin colour and health outcomes related to ultraviolet radiation (UVR) exposure is well established, with light skin colour being associated with increased risk of skin cancers, and dark skin color with increased risk of diseases related to vitamin D deficiency.

“Our finding that people tended to overestimate their skin pigmentation has potentially important implications for the targeting and uptake of skin cancer health promotion programmes.

“If people have an inaccurate perception of their risk, then they are unlikely to respond appropriately to messages about sun protection, which may help to explain why people continue to get sunburned,” Dr Reeder says.

The study was the first to investigate the quality of the skin colour self-report measures commonly used in Australian and New Zealand surveys of UVR exposure. In particular, it established their validity (whether they measure what they are intended to measure), and reliability (whether they can be relied on to produce the same results on different occasions).

“Without establishing how valid and reliable these measures are, the interpretation of survey findings may be misleading.”

Dr Reeder says the research strengthens and extends existing evidence about self- report bias by including a wider range of skin types and ethnicities than previously studied.
The authors and the Cancer Society Social & Behavioural Research Unit received support from the Cancer Society of New Zealand and the University of Otago.