A third of the people who receive federal disability benefits have no income other than their benefit checks. And given their serious medical conditions, their death rate is much higher than that of the general population. In such a vulnerable group, small increases in their disability checks produce big results, finds a new study funded by Social Security, which administers the federal program. For individuals receiving about $10,400 a year in average benefits, for example, an additional $1,000 per year would cut their mortality rate by up to a third of a percentage point in the first four years after their benefits start. The researchers found similar, positive results for disabled workers with families. When the combined benefits paid to…