How many people lie about their age?
The Census Bureau of India makes part of its data public, allowing you and me to fish out interesting facts which they had not thought to look for. The picture above is the age pyramid of India. If you take this at face value, you would think that there are more 70 year olds in India than 52 year olds. Or you would find that the number of people aged 30 is more than the number of 11 year olds. In fact, if you look closely, you would find that there seems to be a major boom in the population exactly every ten years, and a minor boom exactly every five years. Unlikely, isn’t it?
The more likely explanation is that people misreport their age when they become 21 years old, or 26, or 31, and so. They continue to misreport their age for several years. During the ensuing few years they report that they are 20 years old, or 25, or 30, and so on. Children are less likely to do this, but apparently their parents do their lying for them.
One could be charitable and say that people are not very good at mental arithmetic, and therefore round off their age calculations. While this interpretation cannot be ruled out, it is unlikely to be the sole cause of the effect. There is definitely a rounding down of reported age: the number of people reporting their age as 31 is almost a million less than the number who report it as 29. This is more than 9 times the difference which is expected. So, people preferentially reduce their age to the nearest round number. The number of Indians who probably lie about their age could be as high as a hundred and fifty million (150,000,000).
Taking the data about India to be representative of the world’s population, I estimate that at least 10% of the world’s population cheats about their age when they think they can’t be found out.
Note that the population pyramid turns over and narrows at the base. If ages of babies are not misreported widely, then this means that the rate of growth of population has slowed significantly in the last 5 years. This seems to have happened in the rural as well as the urban population. More about this here.