http://trac.syr.edu/immigration/reports/520/

Zero-Tolerance and Family Separations
Family separations, the Administration stated, was the inevitable consequence of prosecuting everyone caught illegally entering this country. As the press widely reported, "he Justice Department can't prosecute children along with their parents, so the natural result of the zero-tolerance policy has been a sharp rise in family separations. Nearly 2,000 immigrant children were separated from parents during six weeks in April and May, according to the Department of Homeland Security."

However, since less than a third of adults apprehended illegally crossing the border were actually referred for prosecution, the stated justification does not explain why this Administration chose to prosecute parents with children over prosecuting adults without children who were also apprehended in even larger numbers. As shown in Table 1, the total number of adults apprehended without children during May 2018 was 24,465. This is much larger than the 9,216 adults that the administration chose to prosecute that month.

Thus, the so-called zero-tolerance policy didn't as a practical matter eliminate prosecutorial discretion. Since less than one out of three adults were actually prosecuted, CBP personnel had to choose which individuals among those apprehended to refer to federal prosecutors. The Administration has not explained its rationale for prosecuting parents with children when that left so many other adults without children who were not being referred for prosecution.

Nor does the zero-tolerance policy explain why so many adults also had their children taken from them who were not prosecuted. For more background, see TRAC's report on the latest case-by-case Border Patrol data.