XPages checkboxgroup and default value

As a follow up to my question on checkboxgroups, how does one set a default value?

Here's the source code for a test XPage:

<?xml version="1.0" encoding="UTF-8"?>
<xp:view xmlns:xp="http://ift.tt/18dEJrC">
<xp:checkBoxGroup id="checkBoxGroup1" defaultValue="First">
    <xp:selectItem itemLabel="First" itemValue="1"></xp:selectItem>
    <xp:selectItem itemLabel="Second" itemValue="2"></xp:selectItem>
    <xp:selectItem itemLabel="Third" itemValue="3"></xp:selectItem>
</xp:checkBoxGroup>
</xp:view>

One would have expected the checkbox with the label "First" to be checked, but not so. Any insight into this would be appreciated.

Thanks!