Final answer:
To increase optimism, adopting a positive explanatory style is beneficial. This involves external, unstable, and specific attributions to negative events, which contrasts with the negative styles linked to depression.
Step-by-step explanation:
People can increase their optimism by adopting a positive explanatory style. According to the research discussed in the Handbook of Positive Psychology by Peterson and Steen, an optimistic explanatory style is characterized by making external, unstable, and specific attributions for negative events, which is in contrast to the internal, stable, and global attributions associated with a pessimistic explanatory style and the onset of depression.
For example, when a student performs poorly on a midterm exam, an optimistic explanatory style would lead to attributions that are external ("The exam was exceptionally hard"), unstable ("I can improve with more study"), and specific ("This was just one difficult subject, not reflective of all my abilities"). Training in such healthy explanatory habits can reduce vulnerability to depression and is an integral part of cognitive-behavioral therapies. Therefore, the answer is 1) Positive explanatory style.