Social security contributions (national insurance in the UK, prispevki in Slovenia, cotisations sociales in France) are mandatory deductions that fund public pension systems, healthcare, unemployment insurance, and disability benefits. Both employee and employer pay contributions — only the employee portion reduces your net salary.

71% in Poland, 22% in France (employee only). Unlike income tax, social security is often a flat rate rather than progressive, though most countries cap contributions above a ceiling income.