HARRISBURG — A University of Washington professor whose survey estimated that more than 1 million registered voters in Pennsylvania lack valid photo identification under a tough new state law testified Thursday that adding steps for people to qualify to vote ensures that fewer will do so.

The professor, Matt Barreto, testified that a long history of research back to so-called Jim Crow laws in the 1930s has been consistent on that point.

“The more hurdles you put before a person, the research only points in one direction,” Barreto said.

Barreto was the eighth witness to testify for the plaintiffs in a challenge to Pennsylvania’s 4-month-old law that will require people who want their votes to count in this year’s presidential election to show eligible photo IDs. The hearing, in its second day, is in front of Commonwealth Court Judge Robert Simpson, who must decide whether to block the law from taking effect in this year’s election as part of a wider challenge to its constitutionality.

The photo ID requirement is a significant departure from current law, which asks only people voting in a ward for the first time to show identification, including such non-photo forms as a utility bill or bank statement.

Women, minorities, the elderly and the youngest voters are those most likely not to have eligible photo IDs under Pennsylvania’s law, Barreto said.

The Department of State, which oversees elections in Pennsylvania, has not produced any kind of study or survey that estimates the number of people without valid photo ID.