This paper reports the results of experiments designed to isolate the impact of various combinations of the following motives on trustworthiness: (i) unconditional other-regarding preferences — like altruism, inequality aversion, quasi-maximin, etc.; (ii) deal-responsiveness — reacting to actions that allow for a mutual improvement by adopting behavior that implies a mutual improvement; (iii) gift-responsiveness — reacting to choices that allow the trustee to obtain an improvement by adopting actions that benefit the trustor; and (iv) vulnerability-responsiveness — reacting to the vulnerability of the trustor by adopting actions that do not hurt the trustor. Our results indicate that — besides unconditional other-regarding preferences — vulnerability-responsiveness is an important determinant of trustworthiness even in cases where the vulnerability of the trustor does not come together with a gift to the trustee. Motivated by our empirical findings we provide formal definitions of trust and trustworthiness based on revealed willingness to accept vulnerability and the response to it.