Who do you trust?

Every year surveys are undertaken to discover which categories of people are generally trusted, and which are not.

Last November Ipsos Mori conducted a poll of a representative sample of people aged 16 and over in Britain, numbering some 1,981, on the topic.

What emerged is that the most trusted groups were
Doctors, - 90% trusted them to tell the truth, and only 6 did not.
They were closely followed by Teachers with 86% and 9 % respectively
Professors were next with 78% and 10% and Judges with 78% and 14%.
Clergy and Priests had 73% and 17%. Beyond them the next group were Scientists with 65% and 22%.
Also above 50% were TV news readers, police, and ordinary people.

The least trusted were
Trade Union Officials with 38% and 45%
Business Leaders with 26% and 62%
Government Ministers with 22% and 71%
Journalists with 18% and 75%
Politicians in general 18% and 76%

So politicians were not trusted to tell the truth by over 3/4 of the sample, and less than 20% actually believe that they tell the truth.

I wonder why people think this? And why when they become ministers are they regarded as slightly more honest?

(Historically, since the poll began in 1993 doctors have topped the list, although in the first year they shared top spot with teachers. The latter seem to slipped a little, but only a little.)