Constant Southworth, the third child of Edward Southworth and Mary Pabodie (Peabody), was born in Duxbury, MA. Since his older brother Thomas was born in 1676 and two younger siblings were born in c.1680 (Mercy) and 1680 (Benjamin), it is safe to assume that he was born about 1678. 
On 10 Feb 1714/15  he married his first cousin Rebecca Simmons,  daughter of John Simmons and Mercy Pabodie (Peabody) his mother's younger sister.
Rebecca, his wife, was named administrator of his estate in 1731.
He and his wife's maternal grandmother was Elizabeth Alden, daughter of John Alden and Priscilla Mullins. Elizabeth was the first child born in New Plymouth.
Initial information for this profile was taken from the Alden Kindred website 
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It may be possible to confirm family relationships with Constant by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
However, there are no known yDNA or mtDNA test-takers in his direct paternal or maternal line.
It is likely that these autosomal DNA test-takers will share DNA with Constant: