except when allocation is off noticeably. Rebalance using contributions or withdrawals: Buy underweighted assets when contributing and sell overweighted assets when withdrawing. This minimizes transaction costs. The contributed or withdrawn amount can be divided across assets in an optimal way that avoids overshooting and minimizes deviation from the target allocation. Also Rebalance Symmetrically whereby allocations across assets or asset categories are traded back to target. Rebalance Asymmetrically whereby allocations are only traded where Assets or categories breach tolerances around a target - this is a Transaction Cost Sensitive approach. Rob Arnott developed a theory of "over-rebalancing". For example, if a portfolio had a target allocation 60/40 split of stocks/bonds, and the allocation shifted to 65/35, over-rebalancing would recommend adjusting to a 55/45 split of stocks/bonds rather than a 60/40 split. His research indicates over-rebalancing might add up to 2% per year, due to the contrarian strategy of buying disproportionately more of temporarily under-valued assets which will eventually recover. See also Asset allocation Merton's portfolio problem References Investment