Those are the UK-derived names. In North America those are all referred to informally as 'professor'. Even the lowest, Assistant Professor, is usually expected

Those are the UK-derived names. In North America those are all referred to informally as 'professor'. Even the lowest, Assistant Professor, is usually expected to run their own lab. Many people don't ever make full Professor. edit: and the jump in pay and responsibility from postdoc to assistant prof is huge - maybe a factor of 2 in pay. The rest is incremental.