If the mother was working all of the year on 2008 they can use her tax return as well, if she did not have a tax return in 2008 then they have to go by her paystubs. What ever way they go it shouls still come out to the same amount if she is making the same as she did in 2008 if she worked.

My soon to be ex refused to do his taxes for 09 and 10 so my lawyer got his W-2's there's always that if there's no tax record's. You want to use the most current record as most people get raises every year.

If there's been a material change in her income, they'll use the current income on her pay stubs. For example, If she got laid off from the good paying job she had in 2008, and took a pay cut when taking a new job, her 2008 earnings wouldn't necessarily be used to calculate support as they don't reflect the current income available to pay support.