Nonmedical (recreational) cannabis use is now legal in many states. Additional states are considering
legalization, despite the weakness of the current evidence base regarding potential impacts on public health.
This R01 proposal seeks 5 years of funding to understand: Aim 1) changes in cannabis use across the first
decade following cannabis legalization; Aim 2) changes in alcohol use, nicotine use, and their co-use with
cannabis following cannabis legalization; and Aim 3) whether psychosocial consequences of cannabis use
change in a legal cannabis context. This knowledge is critical to inform policy, support efforts to maintain hard-
won reductions in teen cannabis use since the 2000s, and promote responsible use by adults. The proposed
project is grounded in life course theories of development and substance use, and uniquely suited to address
stated goals. It continues and expands upon the Seattle Social Development Project-The Intergenerational
Project (SSDP-TIP), which aims to understand the impact of cannabis legalization on youth and parent
cannabis and other drug use (n = 426 families, 80% living in Washington State). SSDP-TIP includes a parent
drawn from another ongoing longitudinal panel study that began in 1985, their oldest biological child, and a
second caregiver (usually the spouse) when available. Seven waves of pre-legalization data (2002-2011; mean
child age in 2011 = 12) and three waves of short-term post-legalization data (2015-2017; mean child age in
2017 = 18) from offspring and parents are available. The proposed study will add three additional annual data
collections post-legalization in 2022, 2023, and 2024 with parents and offspring (mean offspring age in 2024 =
25; range: 11-36 years). For adolescent offspring, the accelerated longitudinal design allows us to compare
offspring measured before legalization to offspring from later birth cohorts measured at the same ages, but
after legalization. This permits the disentangling of child age and policy effects, and facilitates understanding of
the implications of cannabis legalization for prevention in ways that other studies cannot. The inclusion of
young adult offspring allows examination of the impact of cannabis legalization on patterns of use and
consequences across the 20s and into the early 30s. The inclusion of parents permits examination of post-
legalization changes in cannabis and other drug use and consequences across the 40s. SSDP-TIP is the only
study in Washington State with longitudinal data from youth and their parents beginning 10 years before
legalization. With the proposed data collections, it will also stretch to 12 years post-legalization. This
prospective, longitudinal design provides important advantages over large, repeated cross sectional studies by
enabling causal ordering of legalization and changes in behavior, disentanglement of change due to
legalization versus time or age, and study of both within- and between-person change. Findings will inform the
timing of efforts to prevent underage and problematic cannabis, tobacco, and alcohol use and help to clarify
public health impacts in the context of cannabis legalization.
