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Understanding the Evidence on Voting by Mail

Many states expanded voting by mail for the November 2020 election to protect citizens from crowded polling places and COVID-19. There is no evidence to support claims of widespread fraud, but the expected increase in remote voting poses challenges for election officials and voters alike. Experts warn that results of the 2020 election may not be available on election night[1] in larger jurisdictions it may take a week or more[2].

Every state has long permitted voting by mail to accommodate absentee voting. In the 2018 general election, 25 percent of voters cast their ballots by mail[3]. Five states — Colorado, Hawaii, Oregon, Utah, and Washington — already used vote-by-mail as the primary method of voting. But in most states, fewer than 10 percent of ballots were cast by mail in past elections[4]. In these states, local election officials will need to increase capacity and create processes to handle millions of votes by mail: receiving and storing the ballots, then verifying, preparing, scanning, and tabulating the ballots and auditing the results. The process to tabulate ballots takes longer than the typical in-person voting experience of marking a ballot and scanning it.

Reviewing the evidence related to the use of mail-in voting provides key insights into the coming election and potential issues for voters and election officials. 

Should voters worry about voting by mail?

There is no evidence to support claims of widespread voter fraud. Experts and officials do caution that mail ballots may be rejected or not received in time to be counted. If a voter marks a ballot incorrectly at a polling place, the error is often detected, and they are offered the opportunity to correct it. Voting by mail requirements vary by state, but any violation or error may result in the ballot being rejected and not counted.

Some states notify voters when their ballot has been rejected and provide them an opportunity to resolve the issue. Some states employ ballot tracking systems using a barcode, permitting voters to check whether their ballot has been received and accepted[5].

Why are ballots rejected?

In every election, some mail-in ballots are rejected by election officials. Among the reasons ballots are rejected: the ballot was not received on time, the signature on the ballot did not match the signature on file, the ballot lacked a voter’s signature or a witness’s signature, the ballot was missing an important document (such as an affidavit or certification) or included an incomplete document[6].

Approximately 0.96 percent of the ballots mailed back in 2016 were rejected, most because the signature did not match what was on file or the ballot was not signed at all[7]. Signature verification is not performed or required in every state[8]. With the expected increase in voting by mail, and more people voting by mail for the first time, experts anticipate that more ballots will be rejected, as happened during the 2020 presidential primaries[9]. A study in Florida found that younger voters and those who need assistance in casting their ballots, were disproportionately likely to have their mail-in ballots rejected[10]. In Georgia’s 2018 election, younger, newly registered, female, and minority voters were more likely to have their mail-in ballots rejected[11].  

Does it lead to fraud?

Voter fraud of any kind is extremely rare[12]. The Heritage Foundation tracks election fraud cases, its database notes 18 convictions for absentee ballot fraud in 2016 elections[13]. The only significant instances of irregularities with mail-in ballots in recent years were charges of improper collection of absentee ballots in a 2020 city council election in New Jersey, and a congressional election in North Carolina in 2018. There have not been any voting scandals in the five states that hold elections primarily by mail. An analysis of voting in Colorado, Oregon and Washington, found that officials identified 372 possible cases of double voting or voting on behalf of deceased people out of about 14.6 million votes cast by mail in the 2016 and 2018 general elections, or 0.0025 percent[14]. A government commission failed to find instances of widespread fraud in the 2016 election[15].

What impact does it have on voter turnout?

Research indicates that voting by mail slightly increases turnout[16]. Historically, there is no evidence that vote-by-mail benefits any political party[17] [18] [19] [20]. When Colorado switched to all-mail voting, voter turnout increased by 9.4 percent, with increases greatest among lower-propensity voters such as young people, voters with less educational attainment, voters of color and blue-collar workers. The voter turnout for Coloradans categorized as likely Republicans or likely Democrats was almost identical[21]. 

In past elections, people with disabilities[22][23] were especially likely to vote by mail, as were older voters[24][25]Vote by mail increased voting accessibility for groups that may struggle to vote at traditional in-person polling places[26].

How should ballots be returned?

Requirements for when ballots must be received vary by state. Two states, Missouri and Tennessee, require that mail-in ballots be returned by mail[27][28] but in other states voters may deliver their ballot to drop-off locations such as an election office, or drop boxes[29][30]. The US Postal Service recommends  that domestic, non-military voters mail completed ballots at least one week prior to state deadlines[31].

How can people use assistive technology to vote from home?

Some voters, such as those with a disability impacting their ability to mark a ballot by hand, may be offered the option of printing a ballot at home or using their devices to mark ballots. The most secure systems confine vote selection data to the voter’s devices, are not connected to the Internet when selections are made, and remove vote choices from all memory upon closing[32]. Electronic ballot return, the digital return of a voted ballot by the voter, creates significant security risks[33]. Experts recommend that only voters with a disability impacting their ability to mark a ballot by hand should have access to remote electronic ballot marking systems, the most secure option is a pre-printed paper ballot[32].

How can errors or fraud be detected? 

Security features vary by state but include barcodes on ballots and signature verification. Ballot tracking allows election officials to be sure they accept only one ballot per voter. Election audits can also help ensure the integrity of election outcomes, detect errors or fraud, and maintain public confidence in election results. Most states require some sort of post-election audit of votes,[34] which the public can often observe.

Risk-limiting audits provide a more efficient and statistically sound process than typical audits in which a fixed percentage of voting districts or voting machines are examined and compared to the results produced by the voting system. In a typical audit, the same number of ballots are counted whether the election is a landslide or close.

Risk-limiting audits are more like an incremental recount. An initial random sample of ballots is examined based on the margin of victory in an election, the total number of cast ballots, and a predetermined level of certainty that the audit will detect and correct an incorrect outcome. If necessary, additional ballots are examined until there is statistical support for the accuracy of the election outcome[35]. Colorado was the first state to conduct risk-limiting audits statewide, other states are adopting the process or conducting pilots[36]. The National Academies of Sciences, Engineering, and Medicine 2018 report, Securing the Vote: Protecting American Democracy, recommends that states mandate risk-limiting audits prior to the certification of election results[37].

Additional Resources

The Stanford-MIT Healthy Elections Project, led by Nathaniel Persily and Charles Stewart III, assesses and promotes best practices for election officials for the 2020 election. Additional resources and research can be found on

              Vote-By-Mail Resource Guide

              In-Person Voting

              Election Administration Tools

Voting by Mail in the U.S. Past, Present, and Future Advancements by John Curiel, May 20, 2020, The MIT Election Data + Science Lab

U.S. Election Assistance Commission Election Management Resources: Voting by Mail


1. “Fair Elections During a Crisis: Urgent Recommendations in Law, Media, Politics, and Tech to Advance the Legitimacy of, and the Public Confidence in, the November 2020 U.S. Elections,” UCI Law, Ad Hoc Committee for 2020 Election Fairness and Legitimacy, 2020. [Online]. Available:

2. “Election Education and Outreach for Increased Absentee or Mail Voting,” Cybersecurity and Infrastructure Security Agency (CISA) Elections Infrastructure Government Coordinating Council and Sector Coordinating Council Joint COVID Working Group. [Online]. Available:

3. “Election Administration and Voting Survey: 2018 Comprehensive Report,” U.S. Election Assistance Commission, 2019. [Online]. Available:

4. “Voting by Mail in the U.S.,” The MIT Election Data + Science Lab, John Curiel, 2020. [Online]. Available:

5. “The Importance of Accurate Voter Data When Expanding Absentee or Mail Ballot Voting,” Cybersecurity and Infrastructure Security Agency (CISA) Elections Infrastructure Government Coordinating Council and Sector Coordinating Council Joint COVID Working Group. [Online]. Available:

6. “Election Administration and Voting Survey: 2018 Comprehensive Report,” U.S. Election Assistance Commission, 2019. [Online]. Available:

7. “Voting by Mail in the U.S.,” The MIT Election Data + Science Lab, John Curiel, 2020. [Online]. Available:

8.“Voting Outside the Polling Place: Absentee, All-Mail and other Voting at Home Options,” National Conference of State Legislatures, 2020. [Online]. Available:

9. “More Than 550,000 Primary Absentee Ballots Rejected In 2020, Far Outpacing 2016,” NPR, 2010. [Online]. Available:

10. Baringer A. et al, “Voting by Mail and Ballot Rejection: Lessons from Florida in the Age of Coronavirus,” University of Florida Election Science. 2020. [Online]. Available:

11. Shino, E., Suttmann-Lea, M., Smith D. A., “Voting by Mail in a VENMO World: Assessing Rejected Absentee Ballots in Georgia,” University of Florida Election Science, 2020. [Online]. Available:

12. “The False Narrative of Vote-by-Mail Fraud,” The Brennan Center for Justice, 2020. [Online]. Available:

13. “Election Fraud Cases,” The Heritage Foundation. [Online]. Available:

14. “Minuscule number of potentially fraudulent ballots in states with universal mail voting undercuts Trump claims about election risks,’ The Washington Post, 2020. [Online]. Available:

15. “Report: Trump commission did not find widespread fraud,” PBS News Hour, 2018. [Online]. Available:

16. “Colorado 2014: Comparisons of Predicted and Actual Turnout,” Pantheon Analytics, 2017. [Online]. Available:

17. Thompson D. M., Wu J. A., Yoder J., Hall A. B., “Universal Vote-by-Mail Has No Impact on Partisan Turnout or Vote Share” Proceedings of the National Academy of Sciences of the United States of America, June 2020. [Online]. Available:

18. Thompson D. M., Wu J. A., Yoder J., Hall A. B., “The Neutral Partisan Effects of Vote-By-Mail: Evidence from County-Level Rollouts,” Stanford Institute for Economic Policy Research (SIEPR), 2020. [Online]. Available:

19. McGhee E., Romero M., Daly, L., Kousser T., “How Did the Voter’s Choice Act Affect Turnout in 2018?,” New Electorate Project, 2019. [Online]. Available:

20. Barber M., Holbein J. B., “The participatory and partisan impacts of mandatory vote-by-mail,” Science Advances, vol. 6, no. 35, 2020. Availble:

21. Bonica A., Grumbach J. M., Hill C., Jefferson H., “All-Mail Voting in Colorado Increases Turnout and Reduces Turnout Inequality,” National Vote at Home Institute, 2020. Available:

22.  Schur L., Kruse D., “Fact sheet: Disability and Voter Turnout in the 2018 Elections,” Rutgers School of Management and Labor Relations, 2018. [Online]. Available:

23. Belt R., “Contemporary Voting Rights Controversies Through the Lens of Disability,” Stanford Law Review, vol. 68, 2016. [Online]. Available:

24. “Who Votes by Mail?,” The Brennan Center for Justice, 2020. [Online]. Available:

25. “Most Voters Have Positive Views of Their Midterm Voting Experiences,” Pew Research Center: U.S. Politics & Policy, 2018. [Online]. Available:

26. “Research on Vote-By-Mail,” Stanford-MIT Healthy Elections Project. [Online]. Available:

27. “How to Vote,” Missouri Secretary of State, John R. Ashcroft. [Online]. Available:

28. “Absentee Voting,” Tennessee Secretary of State, Tre Hargett. [Online]. Available:

29. “VOPP: Table 9: Ballot Drop Box Definitions, Design Features, Location and Number,” National Conference of State Legislators, 2020. [Online]. Available:

30. “The Use of Ballot Drop Boxes During COVID-19,” Stanford-MIT Healthy Elections Project, 2020. [Online]. Available:

31. “Election Mail,” United States Postal Service. [Online]. Available:

32. Greenhalgh S., Newell S., “Leveraging Electronic Balloting Options Safely and Securely During the COVID-19 Pandemic,” presented at the 10th Annual State Certification Testing of Voting Systems National Conference,  2020. [Online] Available:

33. “Risk Management for Electronic Ballot Delivery, Marking and Return,” FBI, Election Assistance Commission, National Institute of Standards and Technology, and Department of Homeland Security's Cybersecurity and Infrastructure Security Agency, 2020 [Online] Available:

34. “Post-Election Audits,” National Conference of State Legislators, 2019. [Online]. Available:

35. “Knowing It’s Right: Limiting the Risk of Certifying Elections,” Democracy Fund, Tammy Patrick, 2020. [Online]. Available:

36. “Risk-Limiting Audits,” National Conference of State Legislatures, 2020. [Online]. Available:

37.“Securing the Vote: Protecting American Democracy,” The National Academies of Sciences, Engineering, 2018. [Online]. Available:

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