Effects of gun control in Australia

In a previous posting I observed that the homicide rate in New South
Wales fell dramatically following the introduction of gun controls in
1920.

Here, again, is the graph showing the homicide rate in NSW from 1900-1977.
(Vertical scale is homicide rate per 100 000 population)

3      *  
2.8
2.6        
   * 

2.4            * 
2.2  * 

2.0             *  *            *
1.8     *           *   *      *      *           
        * 

1.6                      * 
         *   *  *                 
1.4                *            
 *      *  
   * 
  *   *  
1.2                                *       *       *      *   

1.0   *                   *                     
0.8                    *            
   0         1         2         3         4         5         6         7
   012345678901234567890123456789012345678901234567890123456789012345678901234567

It is quite possible that the decline in the homicide rate was caused
by factors other than gun control (e.g changing demographics, improved
law enforcement, better birth control methods). To test this
hypothesis, we need a control, an adjacent state that did not
introduce gun control in 1920, to see if that state also experienced a
similar decline after 1920. Victoria introduced gun controls into
1921 and Queensland in 1927, so I decided to look at Queensland.

I couldn't find any data for the homicide rate in Queensland prior to
72. I was able to find the numbers of people charged with
manslaughter and murder from 1900-1977. Adding these and dividing by
the population each year gives the homicide charge rate per 100 000
people, which is the vertical scale on the following graph.

9.0 *
8.5
8.0
7.5  * *  *
7.0*  *    *
6.5     *
6.0      *                    *
5.5                  *        |                                      *
5.0                 *      * *|                                *    * *
4.5         **  *             |                 *               * **
4.0           *  ***  * *     |*            *  * *  *  *     **  *     * *   *
3.5                    *  * * | *  *     *** **   *     *  **              **  *
3.0            *       | *    |                      *   **             *     * *
2.5                    |      |  ** *** *          *  *                   *
2.0                    |      |
1.5                    |      |        *
   0         1         2      |  3         4         5         6         7
   012345678901234567890123456789012345678901234567890123456789012345678901234567

The rate did not drop after 1920, but did drop after 1927 (when gun
controls were introduced).

Here is the average homicide charge rate in Queensland for 7 year
periods:

Years average rate
00-06 7.2
07-13 5.0
14-20 4.2
21-27 4.3
28-34 3.0
35-41 2.9
42-48 3.6
49-55 3.2
56-62 4.1
63-69 4.5
70-76 3.4

The rate for 21-27 was the same as the previous 7 year period, but it
dropped significantly in the next seven years.