Children born in the first few months of a sporting season are more likely to make it as professional sportsmen because they will be taller and stronger than their peers. This is the rule called the relative age effect, and for years it has been used to explain why so many professional sportsmen, from ice hockey to football, are born only a few months apart. However, many scientists have said the relative age effect may be overly simplistic, putting too much emphasis on differences at school age. Now, researchers from the University of Essex have found evidence that children born in the autumn may be physically stronger than other children.
The relative age effect was popularised in Malcolm Gladwell’s Outliers. Here Gladwell brings together evidence from studies into various sports that show signs of the relative age effect. The theory is that children born at the start of their school year, in September, will be taller and stronger, and hit puberty earlier, than their peers born later in the year. Because competition at school level is divided into year groups, the September children will never compete with older children. Instead they will stand out as more athletic, stronger and faster that the children born a few months later.
The effect continues into professional sports. Young athletes given the chance to trial for a sports team will be assessed at the start of the season, often around September, and a few months difference in size over their teammates might make all the difference between being selected or dropped. In ice hockey for example, Gladwell quotes research by Canadian psychologist Roger Barnsley who showed that 40% of NHL players are born in the first three months of the season, in this case between January and March.
However, others point out that any competitive advantage a few months of growth might afford a young athlete early on in their career is usually filtered out by the time athletes reach the highest levels.
But research published in 2014 suggests that children born in the autumn are physically stronger regardless of relative age effects. The researchers found that November children were fitter and more powerful that their peers, and September, October and November children were stronger than all others. The children were tested on cardiovascular fitness, handgrip strength and lower body power.
Why would this be? The researchers suggest that mothers exposed to more sunshine towards the end of their pregnancy will benefit from higher levels of bone stimulating Vitamin D. “With children born in the Northern Hemisphere, those born in autumn tend to have slightly bigger bone and muscle mass,” said lead author Gavin Sandercock. “They start off with more muscle, become active earlier, then get involved in athletics sooner. It becomes a positive cycle.”
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