How are Wages Determined When Paychecks are Never The Same?

What is the name of your state? Arizona

If my husband seeks an amendment to his original child support agreement (5 years ago, no one pays, joint physical & legal custody), how do they determine an individual's wages? My husband's paychecks differ every two weeks due to overtime, etc.; and his ex works on commissions as a realtor? Sometimes having little or no income, and at other times brings in $20-30K in a few weeks. ??? Will they base it on previous year's tax returns? Can they take into consideration new spouses/children?

Thanks for all your help

The court will likely expect to see tax returns, W2's and 1099's. New spouses have no obligation when it comes to support except in very special circumstances. Additional children occasionally are taken into account, but the decrease in support is relatively minor.