Set default value radio button in template
Add direct user registration form alter and try to change "1" to 1.

function MYMODULE_form_user_register_form_alter(&amp;$form, &amp;$form_state,$form_id) 
{

  $form['field_gender']['und']['#default_value'] = 1;
    &lt;---or ---&gt; 
  $form['field_personal_details']['und'][0]['field_gender']['und']['#default_value'] = 1;

}