sharepoint - Sharepoint list permission for user to read,create but can only edit their own

Question

I'm trying to allow the user to edit their own list item in Sharepoint Foundation 2010,

1. Open the list that you want to configure.
2. On the Settings menu, click List Settings.
3. On the Customize page, in the General Settings column, click Advanced settings.
4. On the List Advanced Settings page, in the Item-level Permissions section

On my particular list this setting is missing, so I opened a powershell window to do the equivalent

$web = Get-SPWeb YourSite/
$list = $web.Lists["List name"]
$list.WriteSecurity
1
$list.WriteSecurity = 2
$list.Update()
$list.WriteSecurity
1

It does not change to 2 and there is no error. There must be something about this list that makes it different, but I can't find out what it is.