The environment is thought to be a contributory factor.
Compression and displacement of the developing fusion mass by the paraspinal musculature may be a contributory factor.
HPV does not seem to be a contributory factor based on the findings of the current study.
"It might be a contributory factor in many deaths – most of them due to cardiovascular and respiratory conditions.
"We are adding the possibility of a third factor" adding: "BMAA could be a contributory factor in some people".
That seems unlikely to be the whole story (the amount of help needed to compensate would be huge), though it might be a contributory factor.
Alcohol is well known to be a contributory factor in incidents of disorder but it is a complex issue and a crackdown on 'troublesome drunks' is too simplistic an answer.
Silvestre Varela's late winner for Portugal against Denmark on Wednesday actually kept Holland in the tournament for the time being, and yet could ultimately be a contributory factor in their exit.
Aasmah Mir investigates whether a lack of appropriate assistance could be a contributory factor in a number of murders, serious crimes and other incidents involving those with mental health problems.
"As this reinforced and prestressed concrete bridge has been there for 50 years, it is possible that corrosion of tendons or reinforcement may be a contributory factor," he said.
One controversial theory is that an influx of doctors from former British colonies during the 1950s and 1960s, at a time when licensing was not as strict as it is today, could be a contributory factor.
