Source

File: wp-includes/link-template.php

function get_previous_posts_page_link() {
	global $paged;

	if ( ! is_single() ) {
		$nextpage = intval( $paged ) - 1;
		if ( $nextpage < 1 ) {
			$nextpage = 1;
		}
		return get_pagenum_link( $nextpage );
	}
}