Elder son, Lyle, was born Joseph Lyle on 10 January 1968 and his brother, Erik Galen, arrived nearly three years later, on 27 November 1971.

Their father José was extremely controlling of his family, vicious and almost impossible to please. His strict rules covered all aspects of the family's lives, including what they could eat, with whom they could associate and what they were allowed to read. They had to be accountable for their actions for every minute of every day.

Friends later described the brothers as quite different. Lyle was said to have been slightly remote, to have had trouble making the distinction between fiction and fact, but to have had a strong personality and remarkable wit. Erik was apparently sensitive, quiet and kept largely to himself.

The first sign of the brothers' perverse and violent tendencies surfaced when Lyle was 15 and Erik 12. In 1982, their cousin, Diane van der Molen, came to stay with the Menendez family for the summer holidays. It transpired that one night, the brothers and Diane were wrestling playfully, when without provocation or warning, the boys began to remove her clothes. They wordlessly tied her up and took off her shirt but stopped as soon as she screamed.