In Dan Gardner’s excel­lent Risk, he lists psy­chol­o­gist Paul Slovic’s list of 18 fac­tors that influ­ence how we judge the sever­ity of risk:

  1. Cat­a­strophic Poten­tial If fatal­i­ties would occur in large num­bers in a sin­gle event — instead of in small num­bers dis­persed over time — our per­cep­tion of risk rises.
  2. Famil­iar­ity Unfa­mil­iar or novel risks make us worry more.
  3. Under­stand­ing If we believe that how an activ­ity or tech­nol­ogy works is not well under­stood, our sense of risk goes up.
  4. Per­sonal Con­trol If we feel the poten­tial for harm is beyond our con­trol — like a pas­sen­ger in an air­plane — we worry more than if we feel in con­trol — the dri­ver of a car.
  5. Vol­un­tari­ness If we don’t choose to engage the risk, it feels more threatening.
  6. Chil­dren It’s much worse if kids are involved.
  7. Future Gen­er­a­tions If the risk threat­ens future gen­er­a­tions, we worry more.
  8. Vic­tim Iden­tity Iden­ti­fi­able vic­tims rather than sta­tis­ti­cal abstrac­tions make the sense of risk rise.
  9. Dread If the effects gen­er­ate fear, the sense of risk rises.
  10. Trust If the insti­tu­tions involved are not trusted, risk rises.
  11. Media Atten­tion More media means more worry.
  12. Acci­dent His­tory Bad events in the past boost the sense of risk.
  13. Equity If the ben­e­fits go to some and the dan­gers to oth­ers, we raise the risk ranking.
  14. Ben­e­fits If the ben­e­fits of the activ­ity or tech­nol­ogy are not clear, it is judged to be riskier.
  15. Reversibil­ity If the effects of some­thing going wrong can­not be reversed, risk rises.
  16. Per­sonal Risk If it endan­gers me, it’s riskier.
  17. Ori­gin Man-made risks are riskier than those of nat­ural origin.
  18. Tim­ing More imme­di­ate threats loom larger while those in the future tend to be discounted.

For more on risk per­cep­tion, you can do worse than peruse the Wikipedia entry.