Hi All,
Just a quick note about additive versus trigger effects. Per the table on Slide 11 (sorry, I really need to start putting slide numbers on the slides), there are two primary mechanisms through which risk can accumulate: 1) independent and uncorrelated insults and 2) correlated insults. The former references risk that accumulates as a result of various independent exposures . The example that the Ben-Shlomo editorial gives is a road traffic accident, unemployment, and death of a spouse. The three adverse exposures are unrelated, but their sum can result in adverse health consequences. The more common mechanism is correlated insults, which can occur in two forms. The first is risk clustering, whereby adverse exposures tend to cluster together. The Ben-Shlomo editorial suggests various exposures associated with adverse social circumstances, such as low birthweight, poor diet, passive smoke exposure, and fewer educational opportunities. The second is chains of risk, whereby an adverse exposure tends to lead to another, which leads to another, and so on.
Finally I arrive at additive versus trigger effects, which occur in the context of chains of risk. The additive model suggests that each exposure increases the risk of the subsequent exposure AND increases the risk of the outcome itself. Each link in the chain thus increases the risk of outcome in a cumulative fashion. For trigger effects, earlier exposures in the chain increase the risk of subsequent exposures, but they don't independently affect outcome. Only the final link in the chain ultimately precipitates the outcome.
In general, one exposure might increase the risk of the next, but it's likely to do so probabilistically rather than deterministically. E.g., smoking increases the risk of early menopause, but that doesn't mean that smoking always results in early menopause. Chains of risk are chains of probabilities, not definitive occurrences. All part of the complexity of the life course approach. :)
Check out the table on Slide 11 and the Ben Shlomo editorial (from which my explanation is largely sourced) to better understand the hierarchy of these various life course models.
Let me know if there are still questions. Have a great long weekend.
Best,
Rebecca