Another limitation regards the recursive nature of the expressive layer. While it is true that CP and TP can come together to form hierarchical structure, this CP TP structure cannot repeat on top of itself: it is only a single layer deep. This restriction is common to both the expressive layer in humans, but also in birdsong. This similarity strengthens the tie between the pre-existing E system posited to have originated in birdsong and the E layers found in human language.