[pyuser583]

Many countries simply have no way of knowing who their citizens are.

People born abroad are often citizens.

The British have had an insane number of changes in territory that affect citizenship.

Some of their current citizens have citizenship because they were born in Bombay (Mumbai). Others because they were born in Hong Kong.

Maintaining a reliable list of citizens requires regular registration, and loosing citizenship if you don’t register.

[raron]

Even in that case it would probably enough to prove your citizenship once to get into the database.

[pyuser583]

It would have to be combine with other databases. A death index, at the least.