National Insurance number
The National Insurance number is used as a reference number in the Pay As You Earn system, and also by the self-employed. It is also used in applications for Individual Savings Accounts (ISAs), to check that an individual has opened only one ISA in a tax year.
Social Security number
Social Security was originally a universal tax, but when Medicare was passed in 1965, objecting religious groups in existence prior to 1951 were allowed to opt out of the system. Because of this, not every American is part of the Social Security program, and not everyone has a number. However, a social security number is required for parents to claim their children as dependents for federal income tax purposes, and the Internal Revenue Service requires all corporations to obtain SSNs (or alternative identifying numbers) from their employees, as described below. The Old Order Amish have fought to prevent universal Social Security by overturning rules such as a requirement to provide a Social Security number for a hunting license.