Historian Seymour Drescher emphasized the problems of determining whether or not slave-traders were Jewish. He concludes that New Christian merchants managed to gain control of a sizeable share of all segments of the Portuguese Atlantic slave trade during the Iberian-dominated phase of the Atlantic system. Due to forcible conversions of Jews to Christianity many New Christians continued to practice Judaism in secret, meaning it is impossible for historians to determine what portion of these slave traders were Jewish, because to do so would require the historian to choose one of several definitions of "Jewish".