They contradict theories that assert a large causal role for expansion of contraception in the reduction of fertility.

FORBES: UN Population Predictions: Up To 15 Billion by 2100

But diagnosis and the language of biological illness obscure the causal role of factors such as abuse, poverty and social deprivation.

BBC: 'Grief and anxiety are not mental illnesses'

To demonstrate the causal role of language, what's needed are studies that directly manipulate language and look for effects in cognition.

WSJ: Does Language Influence Culture?

LONDON, England (CNN) -- The reality of climate change, and mankind's causal role in the process, are facts that are now almost universally accepted.

CNN: Six degrees of devastation

The movie includes no mention of the mental health issues Tyler faced, and it fails to note that the school district disputes the Long family's account of the causal role bullying played his death.

CNN: SHARE THIS

Because these studies are mainly observational, extensive research is under way to determine the causal relationship between vitamin D levels and disease as well as the role of increasing vitamin D intake.

CNN: How much vitamin D do I need?