Cold War historian John Lewis Gaddis once divided his colleagues into "lumpers" and "splitters". Those in the first camp usually sought to generalise about neat patterns that reduce the untidiness of history, he noted, while the second group preferred to reject these patterns in favour of complexity and endless distinctions. Getting the right balance between lumpers and splitters, wrote Gaddis, was "no easy thing".

It seems a cultural version of the lumper-splitter problem now burdens the painful debate on why a steady flow of young Britons are aspiring to become jihadi brides and henchmen.

Theories from the Left have included cultural insensitivity, the provocation of Islamophobes, marginalisation by the state, victimisation by its security services, villainous Western foreign policy and neo-imperialism, or just Tony Blair.

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