Optimism and pessimism in the context of health: bipolar opposites or separate constructs?

One difficulty plaguing research on dispositional optimism and health is whether optimism and pessimism are bipolar opposites or constitute distinct constructs. The present study examined the Life Orientation Test to determine whether the two-factor structure is explained by method bias (due to measurement) or substantive differences. The authors compared three measurement models: bipolar, bivariate, and method artifact.