For USA taxes, this is partly correct, but not completely.
And this is pretty much just wrong for USA taxes, at least in every company I’ve ever worked with, or any payroll calculation software I’ve ever used or seen.
In fact, the basic assumption is that you will be using the standard deduction, and that will have no itemized expenses. If you do itemize expenses, it’s usually because you have more expenses than the standard deduction, so you would get more money back instead of less.
You should not be under or over withheld just because you do or do not have kids or a house, or and you normally won’t be under-withheld if you have non-standard expenses.
Warning: Long boring post about tax calculation ahead. Do not read this post while operating heavy machinery!
I write payroll programs, and so I’ve written the programs that employers use to calculate the taxes withheld from paychecks. My customers are mostly smaller (up to a few hundred employees) so it’s possible things are different in larger companies, but I don’t think so.
Withholding is calculated based on what you tell your employer on your W4 and a withholding table issued by the taxing authority. For Federal Income Taxes, the tax table is in a booklet called “publication 15”, commonly known as the “Circular E” which is sent by the feds to employers every year, and which includes withholding tables for that year. States that have income taxes have similar booklets they send out to employers in their state.
With the info from the W4 and the tax table, you take the gross wages, subtract the standard deduction different for married and un-married filers), subtract any exemption amount allowed per dependant, and that’s the taxable wage. Find that wage in the table, and there is your tax.
That is for the Federal Income Ta. States have many minor variations in the tables (some have the standard deduction included, some have multiple columns for differing numbers of dependants, etc.) but for the most part they follow this pattern, or they use some scheme like a percentage of the Federal Tax.
If you fill out your W4 form correctly, and do in fact use the standard deduction and file using the same status and dependants as declared on your W4, you’ll come out very close at the end of the year.
So what does cause under-withholding? Mostly things that the other posted mentioned. Your W4 is incorrect or out of date. You get other income that is not subject to withholding. You have a two-income household and have not correctly changed your W4 to compensate (this is tricky, and requires some knowledge of the tax tables to make it come out right), or your employer is simply not following the withholding table correctly. It does happen.
For help figuring out what your put on your W4 (and you can update it any time, just tell your employer your withholding status may have changed, and you want to update your record), here is an online withholding calculator you can use to estimate your taxes.