According to a study, the number of jobs in which firms used bonuses, commissions, or piece rates to tie workers' pay to their performance increased from an estimated 30 percent of all jobs in the 1970s to 40 percent in the 1990s. Why would systems that tie workers' pay to how much they produce have become increasingly popular with firms? The same study found that these pay systems were more common in higher-paid jobs than in lower-paid jobs. Briefly explain this result.