Maybe this tribe has small mutation in FOXP2 gene causing impairment in comprehension of typical human grammar.
EDIT:
Maybe not.
> Gordon ruled out mass retardation. Though the Pirahã do not allow marriage outside their tribe, they have long kept their gene pool refreshed by permitting women to sleep with outsiders.
Unless this mutation is dominant.
> A Pirahã child removed from the jungle at birth and brought up in any city in the world, he said, would have no trouble learning the local tongue.
EDIT:
Maybe not.
> Gordon ruled out mass retardation. Though the Pirahã do not allow marriage outside their tribe, they have long kept their gene pool refreshed by permitting women to sleep with outsiders.
Unless this mutation is dominant.
> A Pirahã child removed from the jungle at birth and brought up in any city in the world, he said, would have no trouble learning the local tongue.
I wonder if they tried that.