public function isAuthor($member = null)
    {
        if (!$member || !$member->exists()) {
            return false;
        }

        $list = $this->Authors();

        if ($list instanceof UnsavedRelationList) {
            return in_array($member->ID, $list->getIDList());
        }

        return $list->byID($member->ID) !== null;
    }