shortcode attribute default value post->ID

I have a shortcode [user-profile] where the author ID is the attribute. I’d like to make it so that if no ID is specified [user-profile] defaults to post author, and when an ID is specified, like [user-profile id="47"], it uses that ID. The shortcode function right now is like this:

function user_profile( $atts, $content = null ) {
	extract(shortcode_atts(array('id' => ''), $atts));
	include ('user-profile.php');
	return $user_hcard;
	}

…which works only when an ID is specified.
(user-profile.php code is here)
How can I set the default to be the $post->ID ?