Article: Moreover, a conflict of interest between professional investment managers and their institutional clients, combined with a global glut in investment capital, led to bad investments by asset managers in over-priced credit assets. Professional investment managers generally are compensated based on the volume of client assets under management. There is, therefore, an incentive for asset managers to expand their assets under management in order to maximize their compensation. As the glut in global investment capital caused the yields on credit assets to decline, asset managers were faced with the choice of either investing in assets where returns did not reflect true credit risk or returning funds to clients. Many asset managers chose to continue to invest client funds in over-priced (under-yielding) investments, to the detriment of their clients, in order to maintain their assets under management. This choice was supported by a "plausible deniability" of the risks associated with subprime-based credit assets because the loss experience with early "vintages" of subprime loans was so low.

Now answer this question: What rationale did asset managers who continued to invest in over-priced investments to the detriment of their clients use?
plausible deniability