Background: In 2013, a study found that mixed breeds live on average 1.2 years longer than pure breeds, and that increasing body-weight was negatively correlated with longevity (i.e. the heavier the dog the shorter its lifespan).
Question: When was a study done that discovered mixed breeds tend to live longer than pure breeds?
Answer:<extra_id_0>.
<extra_id_0> 2013<extra_id_1>