The National Archives version of the document puts a period after happiness, something Allen says was not in the original document and changes the meaning of the second sentence. The sentence is meant to move from the value of individual rights to the importance of government protecting those rights, Allen says. She says this is lost with the addition of the period.
In her study, Allen says the should-be large sentence forms a syllogism, or logical argument based on multiple propositions, and its sections were meant to build upon each other and be parts of the same collective thought. (Via Institute for Advanced Study) She says this style of writing is common in 18th-century literature and the typo obscures "a diverse textual tradition." (Via CAN TV)
The original document in the U.S. Archives shows no period or semicolon, but some argue the period could have faded. (Via U.S. National Archives and Records Administration) Allen also notes that a semicolon, not a period, can be clearly seen in Jefferson's rough draft. (Via U.S. History)
The New York Times points out while the original document has no period, many early copies do. Allen says this may be because the original began fading so quickly, it was hard to copy accurately.
The U.S. National Archives says it's looking into utilizing new imaging equipment to check whether there might ever have been a period after the word "happiness."