For every second born, there must be a first born, but not vice versa. For every third born, there must be a second born and a first born, but not vice versa...

For every second born, there must be a first born, but not vice versa. For every third born, there must be a second born and a first born, but not vice versa... So you're more likely to encounter a first born in the real world than any nth born, since some families will have only had one child.