Final answer:
The percentage of adults in the United States who have never been smokers is not specifically mentioned in the provided reference information. However, it can be inferred that the percentage could be higher than the percentages mentioned for Australia.
Step-by-step explanation:
The question asks for the percentage of adults in the United States who have never been smokers. Unfortunately, the provided reference information does not include the specific answer to this question. However, the reference information does mention that there has been a significant decline in the number of people who smoke in most Western countries. For example, in Australia, the proportion of men and women who smoked in 1976 was 43% and 33% respectively, but in 2014–15, it was 16.9% and 12.1% respectively. This suggests that the percentage of adults in the United States who have never been smokers could be higher than the percentages mentioned for Australia.