Do you want to live a long life? If you're from Iceland, or if you have genes from relatives from this Nordic island nation near continental Europe, then you may well stay on Earth longer than the average person will.

Year after year, Iceland has consistently topped the list of countries with the longest life expectancies. In 2014, for instance, the World Health Organisation (WHO) said a person from Iceland can live up to an average of 81 years old—three years longer than the average life expectancy for people living in the United States.

What's the secret to the longevity of the people from Iceland? Neurologist Kari Stefansson, founder of DeCode Genetics, thinks the staying power of Icelanders has something to do with their genetic makeup.

Stefansson explained that the harsh conditions in Iceland during the early centuries—including widespread poverty, food shortages and unheated homes—may have toughened up the gene pool in the country, so the Icelanders' ancestors passed good DNA for survival to the current generation.

"I don't think it has anything to do with the environment we live in. I don't think it has anything to do with the clean air, the fresh water, or the fish we eat. I think it has all to do with how we select our parents," the neurologist said in an article on NBC News.

"If you look at old people in Iceland, they fall into families," he said. "And these families live in the same clean air, drink the same good water. They eat the same kind of food as the people in families where people die young."

Icelander Stefan Thorliefsson, who is already six months away from being a centenarian, thinks his long life is partly due to his fish diet, which is rich in Omega 3 that is good for the heart.

He agreed, however, that he must have also inherited good genes. I have it from my parents," he told NBC News.