Hai cercato la traduzione di rarer da Inglese a Giapponese

red clay -- a little rarer.
赤い粘土か 珍しいね

despite the fact that actions accruing before 1976 are rarer, they very much exist.
１９７６年以前に発生した 事件は少ないですが 実際に存在します

cavities have become rarer in the developed countries and more people will be able to eat with their own teeth throughout their life.
先進国では虫歯が激減し、自分の歯で一生食べられる人が増えています。