Australia banned lead in gasoline in the early 80s (I remember it happening).
Lead poisoning leads to poor impulse control, aggression etc, all of which are implicated in murder, assault, robbery and other crimes. If you look closely at the crime stats for Australia in those previous replies, you'll see that the homicide rate did indeed drop, but other crimes, such as robberies etc, including armed robberies, not so much. Indeed they stayed fairly constant, and in some cases even rose.
So you could argue that it's a combination of a couple of factors - less brain damaged people and reduced availability of tools to commit murder.
In the US, the proportion of households with guns has been steadily dropping (http://rt.com/usa/gun-ownership-decline-us-111/) so a similar mix of causes would be in effect there.
And as mentioned before, you're 3 times as likely to commit suicide and twice as likely to be murdered if you have a gun in the house. More & more people are wising up to that fact, and that's why the proportion of households with guns is dropping.