This study addresses the counselling of heterosexual men with low-risk
behaviour who, voluntarily or involuntarily, take an HIV test. If suc
h a man tests positive, the chance that he is infected can be as low a
s 50%. We study what information counsellors communicate to clients co
ncerning the meaning of a positive test and whether they communicate t
his information in a way the client can understand. To get realistic d
ata, one of us visited as a client 20 public health centres in Germany
to rake 20 counselling sessions and HIV tests, A majority of the coun
sellors explained that false positives do not occur, and half of the c
ounsellors told the client that if he tests positive, it is 100% certa
in that he is infected with the virus. Counsellors communicated numeri
cal information in terms of probabilities rather than absolute frequen
cies, became confused, and were inconsistent. Based on experimental ev
idence, we propose a simple method that counsellors can learn to commu
nicate risks in a more effective way.