Thirty-three states wrote "Baby Ninth Amendments" into their constitutions, declaring that listing some rights doesn't deny others. State courts have used these provisions to protect the right to earn a living, open schools, refuse medical treatment, give to the homeless, and grow your own food.
The federal Ninth Amendment does the same thing nationally. James Madison framed it to clarify that the Bill of Rights doesn't limit federal power only to what's written. Just because the Constitution lists specific protections doesn't mean people have no other fundamental rights.
The amendment reinforces federalism. It confirms the federal government has only express, limited powers while states and people retain everything else. The Supreme Court in Griswold v. Connecticut in 1965 relied partly on the Ninth Amendment to strike down Connecticut's ban on contraception, finding unenumerated privacy rights.
Justice Antonin Scalia rejected using the Ninth Amendment to create new rights, arguing refusal to "deny or disparage" rights doesn't authorize judges to invent them. Federalism scholars Kurt Lash and Randy Barnett read the amendment to constrain federal power, not expand judicial authority.