Javascript loop problem?

I have the JavaScript function (below) running on my page.

<script type="text/javascript">
function count()
{
	var total = 0;
	var males = 0;
	var females = 0;
	var max = 679;
	alert("max -> "+max);
	for (var idx = 0; idx < max; idx++) 
	{
		idx++;
		if (document.getElementById(idx).checked) 
		{
			total++;
			if (document.getElementById((idx)+"mf").value == "Male") 
			{
				males++;
				
			} else if (document.getElementById((idx)+"mf").value == "Female") 
			{
				females++;
			} 
   		}
		
	}
	alert('total- >'+total+' males->'+males+' females->'+females);
	document.getElementById("num_texts").innerHTML = total;
	document.getElementById("num_males").innerHTML = males;
	document.getElementById("num_females").innerHTML = females;
}

</script>

When I click a checkbox on my page it calls this function. All the checkboxes are clicked by default.

For some reason when I click one checkbox off it counts the total number of checkboxes checked as 339 instead of 678. Anyone know why this could be the case?

There are 679 checkboxes by the way.