the causal mechanism is almost certainly seasonal weather
Not the only possible one. If Alice was born in January and Bob was born in December, she will be 11 months older than him when they start going to school (and their classmates will be in average 5.5 months younger than her and 5.5 months older than him), which I hear can make a difference.
avoid South Hemisphere confounders
The survey already asks for country. Sure, some people will have been born and grown up in a country in a hemisphere other than that they “most identify with” today, but they’ll probably be a small enough minority that they wouldn’t screw up the statistics too much.
If Alice was born in January and Bob was born in December, she will be 11 months older than him when they start going to school (and their classmates will be in average 5.5 months younger than her and 5.5 months older than him), which I hear can make a difference.
I think this way of sorting classes by calendar year of birth might also be six months shifted in different hemispheres (or perhaps vary with country in more capricious ways). IIRC, in Argentina my classes had people born from one July to the following June, not from one January to the following December.
Not the only possible one. If Alice was born in January and Bob was born in December, she will be 11 months older than him when they start going to school (and their classmates will be in average 5.5 months younger than her and 5.5 months older than him), which I hear can make a difference.
The survey already asks for country. Sure, some people will have been born and grown up in a country in a hemisphere other than that they “most identify with” today, but they’ll probably be a small enough minority that they wouldn’t screw up the statistics too much.
I think this way of sorting classes by calendar year of birth might also be six months shifted in different hemispheres (or perhaps vary with country in more capricious ways). IIRC, in Argentina my classes had people born from one July to the following June, not from one January to the following December.
In Massachusetts, USA, the classes of people were from one September to the following August, being based on the day classes started.