How to pre-select items in a select with chosen

How do you load a select with items preselected when the chosen module is installed/enabled?

I’m doing the following:

$my_list = array('item1' = 'item1', 'item2' = 'item2', 'item3' = 'item3');
$form['list_info']['my_list'] = array(
      '#title' => t('Select a list to manage:'),
      '#type' => 'select',
      '#options' => $my_list,
      '#default_value' => $my_list,
  );

But the select shows up empty. Clicking it reveals the items and I can select them, but I need the chosen select to show up with the items selected. How’s this possible?

'#multiple' => true,