The number of hijackings observed have varied over time. The peak period for hijackings occurred from 1968-1973. Dramatic screening measures where implemented at that time, and the frequency of hijacking has been reduced, though not eliminated. On average, from 1972-2001, there are about 22 events per year.

Deaths from hijackings are particularly pronounced in the graph. Peaks are observed for the years 1973, 1977, 1985-86, 1990, 1996, and 2001. These deaths are primarily related to one major event happening in those years (2001 aside). There also appears to be a pattern to these major events, with one every five years or so.

What factors may contribute to this observation?