Nicotine has an affinity for melanin-containing tissue. Darker pigmented people have increased nicotine dependence and lower smoking cessation rates studies have shown. A medical sociologist from Penn State found that African American have a harder time quitting, and suffer more tobacco-related diseases. The reason for this is that melanin pigments, which determine skin color, bind tightly to nicotine. The result from this is that the cancer-causing agents linger and accumulate in other tissues that contain melanin, like heart, lungs, liver and brain.
It's not just people who were born with darker skin, though. Things like tanning and exposure to the sun increase melanin production and increase the same risk factors. The way they found this was by correlating the color of the person's forehead, which is determined by sun exposure and genes. The darkness of the forehead was positively correlated to the number of cigarettes smoked per day as well as nicotine dependence.
A 2008 study of 12,000 people suggested that quitting smoking reduced the chance of others around them smoking. Spouses chances of smoking decreased by 67%, siblings by 25%, and friends by 36%. Interventions to quit smoking with social support did not increase long term cessation rates, though. Also, smokers with major depression disorders are often less successful at quitting than the non-depressed.