Although some politicians are stupid, that's not the main problem.  The reason they lie is because they always have an agenda  and they are mainly motivated by greed and power.

All scientific research is subject to interpretation, and often the same data can be used to draw disparate conclusions.  In climate science for instance, you'll have some scientists who conclude we'll have runaway global cooking due to the greenhouse effect, then others say we'll have an ice age due to the albedo effect.  You'll also have research funded to demonstrate a predetermined conclusion, which particularly in science dependent on statistical analysis can be manipulated through careful choice of sampling methods and groups.

So what can you trust?  Well, if there is a political issue at stake dependent on the conclusions, it's probably not too trustworthy.  Say  something like the safety of a nuclear reactor design.  The folks who want to build it will hire scientists who say it's safe.  Anti nuke greenies will hire scientists who say it's not.  Both will present data to justify their conclusions.  Who do you believe?

Chaos is another problem.  Sometimes very small changes can result in wildly different outcomes.  Economics suffers from this problem because there are so many things going on at once that depend on each other.  Complexity leads to its own set of problems with trusting scientific conclusions.

Taken all together, it's a crapshoot.

RE