Final answer:
Erik Erikson concluded there was weak evidence for different identity achievement routes by gender, focusing on lifelong psychosocial development influenced by social relationships.
Step-by-step explanation:
The theorist who concluded that there was only weak and inconsistent evidence that boys and girls follow different routes to identity achievement is Erik Erikson. Erikson is well-known for his theory of psychosocial development, in which he proposed that an individual's personality develops throughout the lifespan and is influenced by social relationships at different stages. This view contrasts with Freud's perspective that personality is fixed early in life. It was Erikson who emphasized the influence of society and culture on the development of identity, regardless of gender.