Humans in Outer Space

In a recent interview Professor Hawking has said that human kind must colonise space if we are to survive. But he warns that we must be careful about making contact with aliens as they may not be friendly. From the earliest times, when faced with strangers, we've always attacked first and it is only through an equally strong desire to trade that we have, to some extent, overcome our violent natures. So perhaps we aught to send out a warning first - potentially violent but might prove friendly if you've got something we want.