Some timelines are developed from top to bottom, where the oldest element is on top, and the newest is added at the bottom. The examples include forums and message boards, chats, event logs and also Facebook conversations.
Other timelines go the opposite- the newest element is added on top, and as you scroll down you dig deeper in history. This is seen at blogs, e-mail boxes, Facebook timeline and news feed and even in the homepage of Stack Exchange sites.
When should I use the top-to-bottom and when should I use newest-on-top?
I already realize that newest-on-top is very good for situations when you are most interested in the newest items, because current things are more interesting. Despite this tendency, sometimes items are dependent on each-other, and you get a better idea of the whole timeline if you consume it from oldest to newest. Frequently I experience this when I get a new message which is the last of a long thread I wasn't a part of. To understand the thread, I must scroll all the way to the bottom, and start reading from there up.
What should be the leading considerations when deciding the direction of a timeline?
Example: e-mail box with newest message on top
Example: Facebook. News feed has newest item on top, while in conversations it developes downwards.