It helps to be born at the start of the year.
When young people are playing sport at age group levels, those born just before the cut off date for the age limit are at a disadvantage because older players are likely to be bigger and stronger. As a result the older players at each level are more likely to be selected for representative teams, where they have access to better coaching and knowledge to improve their play. This is called the Relative Age Effect, and it has been observed in many sports internationally.
In New Zealand, rugby has been a professional sport since 1996. Was the Relative Age Effect present in both the amateur and professional eras? A Senior Lecturer in Sport, Exercise in Health, Geoff Simons investigated this issue, working with student Luke Adams. Their study included 1051 male athletes who played for the All Blacks, comparing which month of the year they were born in with the New Zealand birth rates per month in the period 1980 to 1994. The results confirmed that the Relative Age Effect was present in both the amateur and professional eras of New Zealand rugby.
Geoff also discovered a statistically significant difference between the amateur and professional eras. The date for age limit selection in New Zealand rugby is 1st January. All Blacks in the amateur era were more likely to be born in the first three months of the year than the New Zealand average, and there was an increase from the amateur to the professional era in the proportion of players born in the first three months of the year. The Relative Age Effect gives a greater advantage to players in the professional era. Of course there are also some players who are very successful despite not having that advantage, for example Richie McCaw, the All Blacks most capped player, was born on 31 December!
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Image credit: "M+MD", used under Creative Commons licence CC BY-NC-ND 2.0