Javascript - cannot dynamically create checked checkboxes

I have the following function that dynamically creates a bunch of checkboxes:

var drawGroups = function(){
  var groups = document.getElementById("groups"); //groups element is a div
  groups.innerHTML = "";

  //groupList is an array containing strings
  for(var i in groupList){
    var groupName = groupList[i];

    var cb = document.createElement('input');
    cb.type = "checkbox";
    cb.checked = true; //this seems to do nothing
    groups.appendChild(cb);

    groups.innerHTML += groupName + "<br/>"
  }
}

Everything I read indicates cb.checked = true should check the checkbox, but it doesn't seem to do anything. How can I create the checkboxes in a checked state?