We take these assessments very seriously, and researched both sides of the argument. We didn't think the monkey owned the copyright -- instead, our assessment was that there's no one who owns the copyright. That means that the image falls into the public domain.

Under US law, for example, copyright claims cannot vest in to non-human authors (that is, non-human authors can't own copyrights). It's clear the monkey was the photographer. To claim copyright, the photographer would have had to make substantial contributions to the final image, and even then, they'd only have copyright for those alterations, not the underlying image.

Because the monkey took the picture, it means that there was no one on whom to bestow copyright, so the image falls into the public domain.