I often think it's comicalSo sang Gilbert and Sullivan's sentry in Iolanthe. Was he right? Are liberals and conservatives born rather than made?
How Nature always does contrive
That every boy and every gal
That's born into the world alive
Is either a little Liberal,
Or else a little Conservative!
I'm no friend of biological determinism, and it is self-evidently obvious that people's political beliefs aren't set in stone at birth. Nor do they remain static throughout their lives (mine certainly haven't). For one thing, it's a cliché that a person who isn't a liberal when they're 17 has no heart and a person who isn't a conservative when they're 40 has no head (this bon mot is usually attributed to Winston Churchill, who actually was a Conservative at 17 and a Liberal at 40). Clearly, an individual's environment, culture and experiences, together with their changing economic interests, will have a major effect on their political outlook.
However, there is evidence that there is some biological basis to political orientation. Neurological research has discovered that different political orientations appear to be associated with different parts of the brain - a conservative orientation with the amygdala and a liberal orientation with the anterior cingulate cortex (see here, here and here). This research is not conclusive, since the structure of the brain is malleable and is itself affected by the patterns of thought that we develop over time. However, other studies, looking at twins and individuals' DNA, do indicate more clearly that political attitudes - though not party affiliations - are in part inheritable (see here and here).
There is a comparison to be made here with twin studies suggesting that religiousness is at least partially inherited too (see here, here and here), though again there is clearly a strong cultural and environmental component at work as well.
Update: See also now here