drupal field list data
Try to add cardinality and settings parameters:

  function _my_installed_fields() {
    $t = get_t();
    return array(
      'my_dropdown_list' =&gt; array(
        'field_name' =&gt; 'my_dropdown_list',
        'label' =&gt; $t('Select an item'),
        'type' =&gt; 'list_text',
        'cardinality' =&gt; 1,
        'settings' =&gt; array(
          'allowed_values' =&gt; array(
            1 =&gt; 'One',
            2 =&gt; 'Two',
            3 =&gt; 'Three',
          ),
        ),
      )
    );
  }