Studies of vulnerable populations,
e. g., children, older adults, exposed to a chronic stressor indicate they

Studies of vulnerable populations, e.g., children, older adults, exposed to a chronic stressor indicate they would exhibit performance decrements and helplessness. This is observed in an experiment wherein the dogs are exposed to different shocks. The dogs eventually learned that no matter what responses, they had no control over the stressor — same case as humans when faced with challenges.

productive most commonly means: achieving or producing a significant amount or result.

explanation:

productive can also mean:

producing or giving rise torelating to or engaged in the production of goods, crops, or other commodities.

these depend on how the word productive is used. most likely productive will mean   achieving or producing a significant amount or result.