Reported violent crimes in Vancouver decreased in the first half of 2011, compared with the same period of time in 2010, according to preliminary figures released by the FBI this week.The decrease in Vancouver reflected the national trend: a 6.4 percent decrease in violent crimes, which include murder, forcible rape, robbery and aggravated assault, the FBI reported. In Vancouver, those crimes were down by 8.3 percent from the 336 reported in 2010.Violent crimes have been declining nationwide for four consecutive years. But for the two previous years, Vancouver’s numbers increased.The decrease in violent crime during the recession has some criminal analysts rethinking the theory that crime goes up when the economy turns south.“Traditional criminology would have taught you that when the economy goesbad, crime goes up,” said John Laws, a crime analyst with the Vancouver Police Department. “We’ve had this past few years of economic downturn and crime has gone down.”Vancouver bucked the national trend when it comes to property crimes, including burglary, larceny-theft and motor vehicle theft. Those crimes decreased 3.7 percent nationwide but increased 7.5 percent in Vancouver, jumping from 3,155 in the first half of 2010 to 3,392 in the first half of 2011. Burglary, in particular, jumped 12.1 percent, from 438 in the first part of 2010 to 491 in the first part of 2011.