Fund managers are constrained in the choice of their investments by concentration limits for their portfolios which are designed to achieve diversification of risk. Despite the impairment in value of wrapped infrastruc-ture debt noted above, the steep decline in the value of certain securities, notably asset-backed securities supported by mortgage loans, has resulted in the relative value of infrastructure debt rising in relation to bond managers' portfolios as a whole. This has produced a mathematical over-weighting of infrastructure assets on a static portfolio basis with the result that some fund managers will be unable to purchase further infra-structure bonds until they rebalance their portfolios to comply with internal diversification requirements.