Baby boomers, who make up a majority of the U.S. teaching force, are inching closer to retirement. Couple that with the downturn in the economy, and renewed worries about pension-fund liabilities are cropping up across the nation. Yet as policymakers focus on ways to make teachers' pension plans sustainable over the long haul, some economists and administrators are concerned about what they see as another cost of those systems: the pressure they exert on the flow of teachers into and out of the profession. Such plans, they say, strongly "backload" benefits toward teachers who stay in the profession for decades, regardless of whether those veterans are the most effective teachers. That bias, critics say, comes at the expense of teachers who are unable to commit to such a long career or who move to teach in another state or district. By tying up precious funds, economists and others suggest, defined-benefit pension plans may be hurting districts' efforts to attract and retain talented novices and midcareer teachers.