A more aggressive posture in Arlington citizens has contributed to the 8 percent drop in the county's serious crime for the first six months of this year over last year, according to police spokesman Barry Hulick.

"People have decided they're just not going to take it," said Hulick. "We definitely do not attribute (the drop) to the weather, the economy or other nebulous factors."

The decline, reflected in statistics released by the police department yesterday, continues a pattern of decreasing crime in Arlington.

Hulick cited the Neighborhood Watch program, in which groups monitor suspicious activities on their streets; Operation Identification, in which citizens etch identification on objects, making them more difficult for burglars to fence, and the habitual offenders program, which gives priority to processing repeat offenders.

Hulick said citizens are more willing to report crimes now and said "the categories of crime have shifted. Some women, for example, have a tendency to fight back more now than in the past, so we might have an increase in purse-snatchings whereas before it might have been a rape or assault."

Aggravated assaults and robbery are the only crimes to increase in the first half of 1983. Assaults jumped 32 percent, from 117 to 155, and robberies 1 percent, from 141 to 143.

Hulick speculated that the increase in aggravated assaults may be due to more willingness on the part of women to report domestic violence.

All other serious crimes dropped, including rape, which decreased 10 percent, from 31 to 28, and murder, which declined 60 percent, from 5 to 2. Burglaries fell 26 percent, from 750 to 558; larcenies by 6 percent, from 2,623 to 2,467, and auto thefts by 7 percent, from 248 to 231.