We do not have any contemporary evidence for the supposed Spartan practice of structural infanticide. No Archaic or Classical Greek source makes any mention of it. The story appears only in Plutarch, who wrote during the 2nd century AD, about 500 years after Sparta had fallen into insignificance. Throughout those five centuries, a fascination with the old glory of Sparta had driven both Greek and Roman authors to circulate increasingly spectacular tales of supposed Spartan customs that had little grounding in reality. The Spartans themselves were all too happy to feed the flames of this myth-making, because it gave them a magnificent and lasting reputation (and brought in the tourist cash).

Earlier sources, such as Xenophon, a great admirer of the Spartans, actually stress that having lots of babies was the foremost duty of all Spartan citizens. They even practiced wife-sharing to increase the birth rate. It was the only way the small and shrinking citizen body could hope to maintain its hold over the vast serf population it was oppressing. Attempts to weed out the weak ones would have been massively counterproductive. In his Constitution of the Spartans, Xenophon talks at some length about the ways in which the Spartans encouraged the fertility of their marriages and the health of their offspring; at no point does he mention any kind of selection process.

Xenophon himself was close friends with one of the most prominent Spartan kings of his day, Agesilaos II (who ruled c. 401-359 BC). Agesilaos was his father's second son; he was short, physically unimpressive, and lame. How could this imperfect human being not only make it through the Spartan education, but even become king of Sparta, if the Spartans routinely disposed of physically flawed babies as unworthy of Spartan citizenship?