US Census

While written in the family bible as Lawrence Joshua Chamberlain, Chamberlain in his autobiography remembered his name as originally being only Lawrence Chamberlain.1
    It is not clear what public records Chamberlain meant when he said "when about to be written on public records, the mother, loyal to her husband's house, wished to prefix... Joshua." The 1850 census was the first to list children, and he was listed as Joshua L. But by this time he was already at college writing his name "J. Lawrence" see History of Bowdoin.

The 1860 census is full of errors, Fanny, who was born in Boston is listed as born in Maine and Joshua, as he is here called, is listed as Massachusetts.
    A lady never admits to her age, and maybe Fanny didn't, she was 34 in July of 1860 and would be 35 one month later.
    The most startling thing recorded here is the third child, Mary. The Chamberlain's did have a new baby that summer, Emily who would not live long.2 It is possible the child listed is Mary Adams, the new daughter of Fanny's adopted Father. The Chamberlain's could have took her in while her mother Helen was sick.

1870 sees a bit better accuracy and also some happy additions, Andrew Tozier, his wife and little Andrew junior were living with the Chamberlains, an arrangement that was likely equally beneficial for all. Besides being able to assist in chores Lawrence was no longer capable of doing Andrew, having formally been a sailor, was a great companion for sailing excursions as can be seen on page 5 of Reunions of the 20th Maine. It is quite an interesting story as to how Tozier ended up living with the Chamberlains, see Andrew Tozier Arrested for Larceny.
1Trulock, Alice R. In the Hands of Providence pg 26.
also familysearch.org
2 Smith, Diane. Fanny and Joshua pg 105
also Emily's birth familysearch.org
Emily's death familysearch.org