|MadSci Network: Development|
Researching this question has been a lot of fun. I looked at the web discussion you referred to ( http://www2.abc.net.au/science/k2/stn-archive1/posts/topic1836.shtm).
The first thing I thought about was confirmation bias: we pay more attention to information that supports our existing ideas, probably because that is where we are directing our attention. This is called a cognitive bias (see http://www.cia.gov/csi/books/19104/art12.html and just about any introductory psychology text). So maybe the person who said that older men’s ears are overly large is falling under the spell of confirmation bias.
Next, I thought about correlation and causation – they are not the same. Just because two things happen at the same time does not mean they cause each other. For instance, deaths in India are correlated with ice cream sales in America. Why? The weather. The two happen at the same time, and so are correlated, but one does not cause the other. Mathematically, a fitted regression line can be drawn by plugging in almost any real numbers to the estimated equation, that is, you can draw a pretty graph of the relationship between nearly any two things, without it meaning anything. We are so used to thinking about relationships between events that we often overestimate. ( http://www.ai.univie.ac.at/archives/Psycoloquy/1999.V10/0026.html, Altman DG, Blan JM. Generalisation and extrapolation BMJ 1998;317:409-410, Kuo Y- H, Extrapolation of Correlation Between 2 Variables in 4 General Medical Journals, JAMA. 2002;287:2815-2817.)
So watching out for bias, I thought about ear and nose growth. Are they different from the rest of the body? Probably not – the outer or external ear is mostly cartilage and skin, with a little fat and muscle and a supply of blood vessels and nerves. Does cartilage and skin continue to grow throughout life? Yes and no. The cells continue to divide, grow, mature and die, just like cells in the gut, liver, bones and elsewhere. And the amount of skin and cartilage in our bodies obviously increases as we grow. But cartilage does not keep growing at the same rate throughout life, and while cartilage keeps dying and being replaced, in the vertebrates it does stop increasing in size in adulthood – which is what is usually meant by ‘keeps growing’. This information can be found in most introductory physiology text books.
(The idea that cartilage mass increases in age may come from the cartilaginous, or boneless, fishes. In the cartilaginous fishes, including sharks, the cartilage ‘skeleton’ does increase in size throughout life. Sharks get really noticeably bigger and bigger ( http://www.isle-of-man.com/interests/shark/growth.htm,). This does not happen in animals with bone skeletons – the cartilage does not keep getting bigger and bigger to any known extent. We would all look very different if it did. We would get big lumps in our noses, ears, thoraxes, shoulders, wrists, knees and ankles – but at least osteoarthritis would not be such a problem! In fact, new cartilage is produced more slowly as people and sharks age (O'Driscoll SW, Saris DB, Ito Y, Fitzimmons JS. The chondrogenic potential of periosteum decreases with age. J Orthopaedic Res. 19(1):95-103, 2001, http://homepage.mac.com/mollet/VBGF/VBGF.html)
The size of the ear canal does not increase with age (Liu TC. Chen YS. Aging and external ear resonance. Audiol. 39(5):235-7, 2000.) The ear canal is defined by skin and cartilage as well as bone. That suggests that there is no overall ear cartilage growth in adulthood.
Does the pinna, or outside flappy part of the ear, grow during adulthood? Do old men really have big ears? Maybe their ears look bigger relative to their head size because they don’t have as much hair. Maybe their ears just get flabbier, and hang down more. Several studies have found that ears are larger in older people. (Ferrario VF. Sforza C. et al., Morphometry of the normal human ear: a cross-sectional study from adolescence to mid- adulthood. J Craniofacial Genetics Developmental Biol. 19(4):226-33, 1999.)
In July 1993, 19 members of the south east Thames faculty of the Royal College of General Practitioners gathered at Bore Place, in Kent, to consider how best to encourage ordinary general practitioners to carry out research. Someone said, "Why do old men have big ears? Some members thought that this was obviously true--indeed some old men have very big ears--but others doubted it, and so we set out to answer the question "As you get older do your ears get bigger?" 206 patients were studied (mean age 53.75, range 30-93, median age 53 years). The length of the left external ear was measured from the top to the lowest part with a transparent ruler; the result (in millimetres), together with the patient's age, was recorded. The mean ear length was 675 mm (range 520-840 mm), and the linear regression equation was: ear length=55.9+(0.22 x patient's age) (95% confidence intervals 0.17 to 0.27). It seems therefore that as we get older our ears get bigger (on average by 0.22 mm a year). (James A Heathcote BMJ 1995;311:1668, 23December)
But there are other reasons than continued growth that could explain this. Maybe fewer people who are old now had plastic surgery for large ears when they were young, and lots of young people are having plastic surgery for large ears. These are alternative explanations. One of the things that does happen is that skin loses elasticity as we age, so it stretches. Ear lobes tend to increase in length with age.
“Another interpretation may be that big ears predict survival: men with smaller ears may die selectively at younger ages. Ear size or pattern, or both, may be a marker of some biological process related to health. However, I don't think that I would go as far as my grandmother, whom I remember admonishing me in early childhood to stretch my ears daily to ensure long life” (Khaw, K-T. Why do old men have big ears? The Chinese believe that long ears predict longevity. BMJ 1996;312:582, 2 March). “An alternative interpretation of the findings is that a secular trend towards smaller ears has occurred during most of the present century. Have the senior citizens in the sample had big ears all their adult lives, and will the younger members keep their smaller ones? If so, what environmental factors, presumably operating during childhood or adolescence, might have been responsible? I wonder whether there has been a steady decline in the boxing or scrubbing of children's ears, or whether big ears are simply another result of passive smoking. This interpretative doubt seems to call for extended pinnametric research: the question can be resolved only by a lifelong follow up study of a cohort of young patients” (Hardisty RM. Lifelong follow up study of young people is needed BMJ;312:582 2 March 1996.)
James A Heathcote went on to say "Why ears should get bigger when the rest of the body stops growing is not answered by this research. Nor did we consider whether this change in a particular part of the anatomy is a marker of something less easily measurable elsewhere or throughout the body."
“All medical research is carried out on selected individuals. . . . . The usefulness of research lies primarily in the generalisation of the findings rather than in the information gained about those particular individuals. . . . .The extent to which it is wise or safe to generalise must be judged in individual circumstance. . . . Many studies use regression analysis to derive a model for predicting an outcome from one or more explanatory variables. The model, represented by an equation, is strictly valid only within the range of the observed data on the explanatory variable(s). When a measurement is included in the regression model it is possible to make predictions for patients outside the range of the original data (perhaps inadvertently). This numerical form of generalisation is called extrapolation. It can be seriously misleading. . . . . To take an extreme example, a linear relation was found between ear size and age in men aged 30 to 93, with ear length (in mm) estimated as 55.9+0.22×age in years. The value of 55.9 corresponds to an age of zero. A baby with ears 5.6 cm long would look like Dumbo.” (Kuo Y-H, Extrapolation of Correlation Between 2 Variables in 4 General Medical Journals, JAMA. 2002;287:2815-2817.)
So do ears keep growing? Who knows. But, thanks to Welsh scientist William Linnard, I know the Celtic words for flap eared: clustlaes. (Prominent ears: a Celtic perspective BMJ 1996;312:582, 2March).
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