Did Donald J. Trump have a fiduciary duty to keep the amount he personally owes in taxes to a minimum?

That was the argument made by the Trump campaign when asked about documents indicating Mr. Trump had declared a $916 million loss on his 1995 income tax returns, possibly shielding him from paying any taxes for 18 years. The campaign said Mr. Trump has “a fiduciary responsibility to his business, his family and his employees to pay no more tax than legally required.”

Simply put, a person who is a fiduciary must act in the best interests of another person, putting that person’s interests ahead of his or her own. But in the context of Mr. Trump’s personal tax returns, legal experts said, the argument that he has a fiduciary duty to others is, well, laughable.