Dual nationality is a "complicated" issue which international football will increasingly have to deal with, England manager Gareth Southgate said.
Declan Rice was named England's squad for the Euro 2020 qualifiers after switching international allegiance from the Republic of Ireland.
Southgate said the issues were "difficult decisions for young men".
"If you are dual heritage then you do feel an affinity to more than one country," he said on Wednesday.
Crystal Palace winger Wilfried Zaha won two caps in friendlies for England before declaring for the Ivory Coast, where he was born.
"We have had players eligible for us who have been with us at under-16s and other countries have given them senior caps very early," Southgate told BBC Radio 5 Live.
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"We have always tried to look at that as ethically as we can.
"There are a couple of players I could have brought into this squad because they are eligible for other countries, but we didn't feel it was the right moment to do that."
England host the Czech Republic on 22 March and are away in Montenegro on 25 March.
Rice, who was born in London with Irish grandparents, picked up three Republic of Ireland caps in friendlies.
Because they were friendlies it left the door open to change allegiance, and his switch was confirmed by Fifa in March.
In a statement in February, Rice said he considered himself "mixed nationality" and that making the decision was "not a clear-cut, simple selection".
Southgate praised both Rice's statement and an article England Women's striker Eniola Aluko wrote on the subject.
"It is a complex situation and it will become more so because the world is more transient," Southgate added.
"The world is a far smaller place in terms of what people are prepared to get up and do, so I think this will become more complicated.
"Whether the rule is the right rule, it's difficult to say."