Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 18621)
+++ wp-includes/query.php	(working copy)
@@ -1730,8 +1730,7 @@
 				$in = ($cat > 0);
 				$cat = abs($cat);
 				if ( $in ) {
-					$q['category__in'][] = $cat;
-					$q['category__in'] = array_merge( $q['category__in'], get_term_children($cat, 'category') );
+					$q['category__in'][] = $cat; // children are included by default in get_tax_sql()
 				} else {
 					$q['category__not_in'][] = $cat;
 					$q['category__not_in'] = array_merge( $q['category__not_in'], get_term_children($cat, 'category') );
@@ -1746,7 +1745,6 @@
 				'taxonomy' => 'category',
 				'terms' => $q['category__in'],
 				'field' => 'term_id',
-				'include_children' => false
 			);
 		}