Sissy Spacek -- Good Genes or Good Docs?

Sissy Spacek's good looks are nothing to laugh at!

Here's a 21-year-old-version of the "Carrie" star at Sardi's back in 1970 (left) and 46 years later ... the now 67-year-old mother of 2 at a movie premiere in Culver City earlier this year (right).

"It's beautiful."

The question is ...