Genealogists tend to focus very closely on questions of evidence. The reason is very simple. Many apparently sound family trees are riddled with inconsistencies, leaps of illogic and undocumented assertions. It is all too easy to waste weeks researching non-existent ancestors before uncovering the flaws in such pedigrees.
Given the nature of much genealogical evidence, with garbled family stories, ludicrously repetitive naming traditions and half-obliterated parish registers, absolute certainty is often impossible. The best a researcher can aim for is a well-reasoned argument that takes account of any surviving records or traditions, and assesses probabilities as dispassionately as possible. Even then, the pattern-seeker’s trap awaits: if you stare at gibberish long enough, it will start to look intelligible.
Take the Irish ancestry of Ronald Reagan. The earliest documented ancestor is Michael Regan, who married in England in 1852 and recorded his father as Thomas. The English 1851 census (very fortunately) gave Tipperary as his place of origin and his age as 21. So far, so good. And the researchers who searched Tipperary parish registers did indeed find a Michael, son of Thomas Regan, baptised in Ballyporeen in 1829.
But at least 20 of the 53 Catholic parishes of Tipperary have no records for the years around 1830. Both Michael and Thomas are unimaginably common forenames and there were more than 50 Regan households in the county at the time. Even for parishes that have records, five minutes on rootsireland (which only covers two-thirds of the county) will get you at least ten Thomas Regans baptising children over the period. So it is perfectly likely, probable in fact, that more than one Michael, son of Thomas, was baptised in the relevant period. A very slender basis on which to build the Ronald Reagan Visitor Centre.
However good the documentation, however impeccable the reasoning, humility and scepticism are always required. In the words of science writer Jonah Lehrer, just because something is true doesn’t mean it can be proved.
And just because it can be proved doesn’t mean it’s true.