This doesn't exactly answer the question but for anyone using AngularJS trying to achieve this, the answer is slightly different. And actually the normal answer won't work (at least it didn't for me).
Your html will look pretty similar to the normal radio button:
<input type='radio' name='group' ng-model='mValue' value='first' />First
<input type='radio' name='group' ng-model='mValue' value='second' /> Second
In your controller you'll have declared the mValue that is associated with the radio buttons. To have one of these radio buttons preselected, assign the $scope variable associated with the group to the desired input's value:
$scope.mValue="second"
This makes the "second" radio button selected on loading the page.
EDIT: Since AngularJS 2.x
The above approach does not work if you're using version 2.x and above. Instead use ng-checked attribute as follows:
<input type='radio' name='gender' ng-model='genderValue' value='male' ng-checked='genderValue === male'/>Male
<input type='radio' name='gender' ng-model='genderValue' value='female' ng-checked='genderValue === female'/> Female
