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Planners always use growth curves, just like the ones you show (18 October, p 40) , and they are always wrong. No growth is indefinite. In the early 1930s telephony growth was phenomenal. There were published concerns that by the 1950s every female in the UK would have to be a telephonist, just to keep …

Eric Solomon asks whether the anomalies described in Marcus Chown's article (20 September, p 38) might be explained by approximation errors in iterative calculations (18 October, p 21) . Not only is approximation error unlikely to be the explanation for a discrepancy so consistent that it can now be predicted in advance with a degree …

Feedback revisits the placebo effect (18 October) that you discussed earlier (23 August, p 36) . Doctors used to visit patients in their homes: the request to call on a very sick toddler was common enough. Not infrequently one was greeted by a lively toddler and a very relieved if somewhat apologetic mum. Yes, the …

Paul Collins, reviewing Bee Wilson's excellent book Swindled , concludes that "our Accums will always be busy" (13 September, p 47) . As a public analyst I could lay claim to being a present-day Friedrich Accum: my job is to direct the chemical analysis of food to see whether it is safe, is as described …

I was stunned by your recent article announcing that 61 Nobel laureates endorsed Barack Obama for president (11 October, p 50) . If 61 such august people could get together for something as meaningless as a presidential election, I would hope their efforts would yield something more beneficial than just an endorsement for someone else …

It was intriguing to read Michael Brooks's interpretation of how people may have been affected by visiting a temple in pre-Columbian South America (6 September, p 37) . He describes being led through a dark, confusing building, where limited light illuminates distorted images, while being bombarded with unearthly music that seems to come from everywhere. …

If tools maketh the monkey, by extending the self and thus creating an awareness of self (11 October, p 42) , then surely by extension the same would be true of clothing and self-adornment, another crucial phase in human development. Could this be the next stage in Atsushi Iriki's investigation: macaques in lipstick?

• We mistakenly reported that all 59 people who received a new treatment for their inoperable brain cancer had died (4 October, p 16) . We are pleased to report that several patients in the trial by Transmolecular of Cambridge, Massachusetts, are still alive. • We said "marine organisms use dissolved CO 2 in the …

Many thanks for your refreshingly candid review of the madnesses of the current economic system (18 October, p 40) . I am left, however, asking whether the necessary transition can ever be made under a representational democracy. While many people are concerned and may be prepared to make some short-term sacrifices for long-term goals, will …

I read with interest Roger Waller's ideas for steam powered cars (4 October, p 37) . However, given that he is talking about tanks holding steam at a temperature of 400°C and 120 atmospheres pressure, I am surprised he made no mention of safety issues. At first thought there seems to be quite a bit …