What were the primary drivers of increases or decreases in border crossings from 2016 to 2020?

1. The geopolitical and economic push: violence, state fragility, and shifting origins

A primary driver was worsening conditions in Central America and beyond — gang violence, local state failure, and economic collapse in countries such as Venezuela pushed many to migrate, altering the origin mix away from Mexico toward Northern Triangle and other countries and contributing to higher encounters by late 2018–2019 ^{[3] [1] [6]}. Migration experts and the Penn Wharton analysis point to interstate and internal armed conflict and fragile economies as central push factors that began well before 2019 and helped fuel the surge observed that year ^[3].

2. Mexico’s changing role and long-term demographic shifts

At the same time, longer-term changes in Mexico — economic growth and falling fertility — reduced traditional northward flows and changed the character of crossings; by the mid‑2010s non‑Mexican migrants increasingly outnumbered Mexicans, shifting routes and pressures on the U.S. Southwest border ^[1]. Migration Policy notes that reduced circular migration from Mexico following economic shifts depressed Mexican-origin crossings even as overall encounters later rose because of other nationalities ^[1].

3. U.S. enforcement, deterrence, and transactional policies under two administrations

Stronger enforcement and structural shifts in U.S. policy also shaped counts: apprehensions fell to historic lows in 2017, then surged by 2019 amid enforcement changes and messaging that altered migrant decisions, while border management measures like increased removals and processing differences for families versus single adults affected where and how people attempted crossings ^{[2] [7]}. Critics and proponents differ on causation: some argue tougher enforcement reduced net entries, while others note that release policies and limited interior removals functioned as pull factors, complicating the attribution ^{[1] [8]}.

4. Measurement artifacts and the role of repeat attempts

Statistical quirks mattered: encounter data count apprehensions and expulsions, not unique individuals, so repeat crossings — especially common under policies that expelled people quickly — inflated encounter totals and complicate year‑to‑year comparisons ^{[4] [5]}. Analysts warned that policies like Title 42, which began in March 2020, produced higher recidivism and repeat encounters that make it harder to read the data as a straight trend in migrant intent ^{[4] [5]}.

5. The pandemic shock: a sudden policy and mobility break in 2020

The COVID‑19 pandemic sharply interrupted flows and transformed enforcement: travel restrictions, halted asylum processing, and Title 42 expulsions led to an abrupt fall in recorded crossings in early 2020 even as underlying drivers persisted — the pandemic created an artificial trough whose effects ripple through the counts ^{[3] [9]}. BBC and Penn Wharton reporting confirm that encounters dropped with pandemic measures but that the underlying surge trajectory that began before 2020 was not wholly erased ^{[6] [3]}.

6. Competing narratives, data politics, and what the numbers cannot say

Interpretation is contested: political actors cite declines or spikes to support arguments about open borders or successful enforcement, yet academic and nonpartisan analysts emphasize mixed causes — push factors, demographic change, operational success at apprehending more people, and data artifacts like recidivism — and caution against reading encounters as a one‑to‑one measure of unique migrant arrivals ^{[8] [5] [2]}. Reporting and policymaking incentives can bias which drivers are foregrounded: enforcement agencies emphasize operational metrics, while migration experts highlight root causes abroad ^{[5] [1]}.