Adding Variables in Javascript

I created a function to add values that are entered into text boxes and then writes those added values to a text box in the parent document. Instead of adding the values it concatenates them and writes the concatenated numbers in the parent document. What am I doing wrong? Here is the function code:

var addTotal = 0
function updateParent(partAmt){
var subTotal = 0

subTotal = partAmt.value
addTotal = addTotal+subTotal
opener.document.transact.tot_pay.value = addTotal
}

The 3 text boxes are coded like this:

<input type="text" name="cc_amount" onBlur="updateParent(this)">
<input type="text" name="ck_amount" onBlur="updateParent(this)">
<input type="text" name="o_amount" onBlur="updateParent(this)">

Form values are strings; adding strings concatenates them. Try this:

var addTotal = 0;
function updateParent(partAmt) {
addTotal += partAmt;
opener.document.transact.tot_pay.value = addTotal;
}

<input type="text" name="cc_amount" onBlur="updateParent(eval(this.value))">

Might as well pass the number (eval()ed field value) since that's what your function needs.