Indirect genetic effects (IGEs) are the basis of social interactions among conspecifics, and can affect genetic variation of non-social as well as social traits. We used flour beetles (Tribolium castaneum) of two phenotypically distinguishable populations to estimate genetic (co)variances and the effect of IGEs on three life-history traits: development time (DT), growth rate (GR), and pupal body mass (BM). We found that GR was strongly affected by social environment with IGEs accounting for 18% of the heritable variation. We also discovered a sex-specific social effect: male ratio in a group significantly affected both GR and BM. That is, beetles grew larger and faster in male-biased social environments. Such sex-specific IGEs have not previously been demonstrated in a non-social insect. Our results show that beetles that achieve a higher BM do so via a slower GR in response to social environment. Existing models of evolution in age-structured or stage-structured populations do not account for IGEs of social cohorts. It is likely that such IGEs have played a key role in the evolution of developmental plasticity shown by Tenebrionid larvae in response to density. Our results document an important source of genetic variation for GR, often overlooked in life-history theory.