6. What About Twins?
Twins are a separate unit in the birth order psychology system. Their birth order only relates to each other — not other siblings — one takes the oldest role and one takes the youngest. Identical twins tend to be more equal than fraternal twins.
7. The Gap
If there is a space of four or more years between siblings everything resets and the next child functions as the oldest or an only child of he or she is the last. Donald Trump is a particularly in your face example — he is the fourth of five children but there’s a four year gap between him and the next-oldest sibling.