Should a 14-year-old married girl who migrates to Europe be viewed as a child - or a spouse?
The issue has put European governments in a spin: forcing a policy U-turn in Denmark, new legislation in the Netherlands and an agonised debate in Germany.
Analysts say early marriage is often carried out in refugee camps in Lebanon, Jordan and Turkey by families trying to protect girls from poverty or sexual exploitation. Elsewhere, poor families might marry off their young daughters in exchange for dowries.
The question is one of rights and protections - but which? When authorities stop minors cohabiting with their older spouses, are they combating child abuse or breaking up (often already traumatised) families?
Depending on where you go in Europe, you'll find a radically different range of responses to the issue.
Denmark's response has swung first one way and then the other.
In February, Integration Minister Inger Stojberg vowed to act after a review found dozens of cases of girls living with older men in asylum seekers' accommodation - which the minister called "totally unacceptable".
Couples would require "exceptional reasons" to live together below the age of 18 (the legal age for marriage in Denmark) and no cohabitation would be allowed whatsoever if one party was below 15.
But separation reportedly prompted two migrants under 18 to attempt suicide.
The policy was reversed earlier this week - with children as young as 14 reunited with their husbands - after the issue was raised with the Danish Immigration Service (DIS) by lawmaker Josephine Fock.
"It is completely outrageous. We are talking about people who have fled to Denmark who are being split from each other. Some of them have children together and investigating individual [asylum] cases takes an unbelievably long time," Ms Fock told Metroxpress news service.
The DIS cited Denmark's "international obligations" as the trigger for its policy change, concluding that enforcing separate living quarters would violate the UN's Convention on the Rights of the Child and Article 8 of the European Convention on Human Rights, which guarantees the right to one's "private and family life".
That has prompted conservative politicians to call for Denmark's withdrawal from such treaties.
In the Netherlands, policy has shifted in the other direction - with the government moving swiftly last year to close a legal loophole which allowed child brides to live with older husbands in asylum centres.
And politicians have grappled with the same dilemma elsewhere in Europe - though on the whole each country is dealing with just a handful of cases.
The issue takes on much broader significance in Germany, which has greeted some 1.2 million migrants since last year under Chancellor Angela Merkel's "open-door" policy.
Here the authorities' response has been inconsistent and, some claim, confused.
Data suggest that in Germany there are at least 1,000 marriages where one or both parties are under the legal marriage age of 18, of which more than half are in the southern state of Bavaria.
Legal marriage or state-sanctioned abuse?
The official confusion is reflected in one reported case: a 15-year-old Syrian girl married to her 21-year-old cousin. She was first separated from him in the city of Aschaffenburg, Bavaria, for reasons of child protection.
Her husband lost an appeal to a family court, but the decision was eventually set aside by a regional court, which judged that the marriage should be recognised as it was legal in the country of origin.
But the city appealed, and the pair are now awaiting a judgment from Germany's federal court.
In response, Germany's justice ministry has set up a working group to agree a consistent response.
Ironically, the Family Affairs Minister Manuela Schwesig cited the UN Convention on the Rights of the Child to argue against under-age cohabitation, claiming that violated children's rights to "play, education and health".
And the issue only threatens to become more pressing, despite the efforts of global campaign groups to eliminate child marriage - which they claim in many cases is in fact forced marriage.
Unicef figures from the vast Syrian refugee camps in Jordan suggest the proportion of registered marriages where the bride was under 18 rose from 12% in 2011 (roughly the same as the figure in pre-war Syria) to 18% in 2012, and as high as 25% by 2013.
And Jordan's Chief Islamic Justice Department was recently quoted as saying child marriages represented about 35% of all marriages of Syrian refugees in 2015.
"There are a number of reasons why families are opting for child marriage for their daughters," says charity Save the Children.
"As refugees, Syrian families are reliant on dwindling resources and are lacking economic opportunities. At the same time, they are all too aware of the need to protect their daughters from the threat of sexual violence."